How to Reduce PDF File Size - The Complete Guide

Large PDF files cause a variety of problems. The most common issue is email attachment size limits. Most email services cap attachments at 25MB, making it impossible to send files that exceed this limit.

Websites and online services also impose upload limits. University assignment portals, government service platforms, and similar systems typically allow only 5-10MB per file. File size issues frequently arise when submitting portfolios or cover letters for job applications as well.

Large files also consume significant storage space. Free tiers on cloud storage services like Google Drive and Dropbox are limited, so unnecessarily large files waste precious space.

Additionally, large PDF files open slowly and are sluggish to scroll through, resulting in a poor user experience. This is especially noticeable on mobile devices, where performance suffers and data usage increases.

With proper compression, you can reduce file size by 50-90%, and in most cases the quality loss is barely noticeable.

Before choosing a compression method, it helps to understand why the file is large in the first place.

Main causes:

1. High-resolution images

- Scanned documents are typically 300 dpi or higher

- Photos taken with a digital camera

- Each image can occupy several megabytes

2. Uncompressed images

- Original images embedded without compression

- PNG or BMP format instead of JPEG

- Image quality higher than necessary

3. Embedded fonts

- All fonts used in the PDF are stored within the file

- Using many specialty fonts increases file size

- Korean (CJK) fonts are significantly larger than Latin fonts

4. Metadata and annotations

- Edit history and layer information

- Comments, bookmarks, and form data

- Residual data from hidden content or deleted pages

5. Inefficient PDF creation

- PDF converted without optimization

- Multiple PDFs merged together

- Older PDF format versions (PDF 1.3 or earlier)

Identifying the root cause of a large file helps you choose the most effective compression approach.

The simplest and fastest approach is to use an online tool. No software installation is needed, and they work on all operating systems.

Recommended online tools:

1. Smallpdf (smallpdf.com/compress-pdf)

- The most popular PDF compression tool

- Simple drag-and-drop upload

- Automatically applies optimal compression

- 2 free files per hour (no login required)

- Files are automatically deleted 1 hour after compression (for security)

- Also supports compression of Word, Excel, PPT files

2. iLovePDF (ilovepdf.com/compress_pdf)

- Completely free with no usage limits

- Choose compression level (low / medium / high)

- Batch processing available (compress multiple files at once)

- Additional features: merge, split, convert PDFs

- Available in multiple languages

3. PDF24 Tools (tools.pdf24.org/compress-pdf)

- German company with strong privacy protections

- Fine-grained compression ratio adjustment

- Option to process files without uploading to server

- Completely free with minimal ads

4. Adobe Acrobat Online (adobe.com/acrobat/online/compress-pdf)

- Tool from the official creators of PDF

- Best-in-class compression algorithm

- 2 free files per day

- Login required

5. PDF.io (pdf.io/compress)

- Fast processing speed

- No file size limits

- Clean interface

How to use (general steps):

1. Visit the website

2. Select your PDF or drag and drop

3. Start compression (automatic or choose options)

4. Download the compressed file

5. Your file is automatically deleted after 1-2 hours

Advantages of online tools:

- No installation needed, ready to use immediately

- Works on all OS (Windows, Mac, Linux)

- Automatically applies the latest compression technology

- Free to use

Cautions:

- Avoid uploading sensitive documents online

- Requires an internet connection

- Very large files may take a long time to upload

Here are several methods for Windows users.

[Method 1] Microsoft Edge Browser (Free, Built-in)

You can easily compress PDFs using Edge, which is built into Windows 10/11.

1. Open the PDF in Edge (right-click the file > Open with > Microsoft Edge)

2. Press Ctrl + P (Print shortcut)

3. Printer: select 'Microsoft Print to PDF'

4. Expand 'More settings'

5. Quality: select 'Low' or 'Medium'

6. Click 'Print'

7. Save with a new filename

Compression: typically 30-50% reduction

Pros: no program installation needed

Cons: compression ratio is not high

[Method 2] Adobe Acrobat Pro (Paid)

The most powerful and professional option.

1. Open the PDF in Acrobat Pro

2. File > Save As Other > Reduced Size PDF

3. Choose compatibility level (newer versions achieve better compression)

4. Click 'OK'

Alternatively:

1. File > Print > Adobe PDF

2. Advanced Settings > PDF Settings > Smallest File Size

3. Print

Compression: up to 80-90% reduction

Pros: adjustable quality, professional results

Cons: paid subscription ($12.99/month)

[Method 3] PDF-XChange Editor (Free/Paid)

A cost-effective PDF editor.

1. Download and install PDF-XChange Editor

2. Open the PDF file

3. File > Optimize PDF

4. Choose a preset: 'Screen View', 'Web', or 'Print'

5. Adjust image quality

6. Click 'Optimize'

Compression: 60-80% reduction

Pros: the free version is powerful

[Method 4] NXPowerLite (Dedicated Compression Tool)

Optimized for batch compression.

1. Install NXPowerLite Desktop (free trial available)

2. Drag and drop your PDF files

3. Select a profile (screen, mobile, low quality)

4. Click 'Optimize'

5. Original is backed up; compressed version replaces it automatically

Compression: up to 95% reduction

Pros: batch processing, automation-friendly

Cons: paid ($49.95)

Effective compression methods for Mac users.

[Method 1] Preview App (Free, Built-in)

You can compress PDFs using the Preview app that comes with every Mac.

1. Open the PDF in Preview (double-click)

2. File > Export... (Shift + Cmd + E)

3. Click the 'Quartz Filter' dropdown

4. Select 'Reduce File Size'

5. Click 'Save'

Compression: 50-70% reduction

Pros: the simplest and fastest method

Cons: compression can be aggressive and may reduce quality noticeably

If quality loss is significant:

1. Open the Automator app (in the Applications folder)

2. Create a new 'Quick Action'

3. Add the 'Scale Images' action

4. Set custom compression settings (recommended: 150 dpi, JPEG quality 80%)

5. Save the workflow

6. Right-click your PDF > Quick Actions > select your saved action

[Method 2] ColorSync Utility (Free, Built-in)

A built-in Mac tool offering finer control.

1. Open ColorSync Utility (Applications > Utilities)

2. Drag your PDF file into the window

3. Click the 'Filters' dropdown

4. Select 'Reduce File Size' or create a custom filter

5. Click 'Apply'

Creating a custom filter:

1. ColorSync Utility > Filters tab

2. Click the '+' button to create a new filter

3. Image sampling: 300 dpi -> 150 dpi

4. Image compression: JPEG quality 80%

5. Save and apply the filter

Compression: 60-80% (adjustable)

Pros: quality control

Cons: complex interface

[Method 3] PDF Expert (Paid)

One of the best PDF apps available for Mac.

1. Install PDF Expert

2. Open the PDF file

3. File > Reduce File Size

4. Select a quality preset

5. Save the reduced file

Compression: 70-85% reduction

Pros: easy to use with excellent output quality

Cons: paid ($139.99, but free trial available)

[Method 4] Homebrew + Ghostscript (Free, Advanced)

A command-line method for power users.

1. Install Homebrew (brew.sh)

2. In Terminal: brew install ghostscript

3. Compression command:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

Compression: depends on user settings

PDFSETTINGS options:

- /screen: 72 dpi (maximum compression, for on-screen viewing)

- /ebook: 150 dpi (medium compression)

- /printer: 300 dpi (print quality)

- /prepress: 300 dpi (for publishing)

Pros: most powerful with fully customizable settings

Cons: requires technical knowledge

How to choose the optimal compression settings for your situation.

Recommended settings by use case:

1. Email attachments (target: 5-10 MB)

- Image resolution: 100-150 dpi

- JPEG quality: 60-70%

- Font embedding: only what is necessary

- Recommended: medium to high compression

- Documents viewed on screen can tolerate quality reduction

2. Website uploads (target: 10-20 MB)

- Image resolution: 150 dpi

- JPEG quality: 70-80%

- Recommended: medium compression

- Balance between download speed and visual quality

3. Print use (quality first)

- Image resolution: keep at 300 dpi

- JPEG quality: 90-100%

- Recommended: low compression or lossless

- Quality matters more than file size

4. Archive / long-term storage

- Preserve the original as much as possible

- Use lossless compression only

- Retain metadata

- Completeness takes priority over compression ratio

5. Mobile viewing

- Image resolution: 100 dpi

- JPEG quality: 60-70%

- Recommended: high compression

- Differences are barely visible on small screens

Pre-compression checklist:

- Have you backed up the original file?

- Have you considered the intended use after compression?

- Do you plan to print the document?

- Does text need to remain selectable?

- Is color accuracy important?

Post-compression checklist:

- Is the text still readable and not corrupted?

- Are the images clear (not blurry)?

- Are all pages rendering correctly?

- Do hyperlinks still work?

- Did you achieve your target file size?

Typical compression ratios for reference:

- Low compression: 20-40% reduction

- Medium compression: 40-70% reduction

- High compression: 70-90% reduction

If the file is still too large after compression:

1. Delete unnecessary pages

2. Extract images, re-compress them separately, then re-insert

3. Split the PDF into multiple files

4. Convert images to grayscale (if color is not needed)

Q. Does compressing a PDF significantly reduce image quality?

A. With appropriate settings, it is difficult to notice any difference with the naked eye. For documents you will view on screen, 150 dpi and JPEG quality 80% is perfectly sufficient. 300 dpi is excessive for anything other than printing.

Q. Can a text-only PDF be compressed?

A. Yes, text-only PDFs can also be compressed. The process involves subsetting embedded fonts, removing duplicate resources, and updating the PDF to a newer version for optimization. Reductions of 20-40% are typical.

Q. Can I compress an already-compressed PDF again?

A. You can, but it is largely ineffective. Re-compressing an already-compressed image only degrades quality without meaningfully reducing the file size. It is best to compress from the original in a single, well-chosen step.

Q. Can I compress a password-protected PDF?

A. Most tools require you to remove the password first. If you know the password, Adobe Acrobat Pro allows you to open and compress the file, and you can re-apply a password afterward.

Q. Are online tools safe? Could my documents be leaked?

A. Leading online tools use SSL encryption and automatically delete files within 1-2 hours of processing. That said, for sensitive documents (contracts, personal information, etc.), it is recommended to use offline software.

Q. Is it possible to compress a PDF to under 300 KB?

A. It depends on the original size and content. Files with many images are difficult to compress that small, while text-heavy files are more achievable. Converting images to grayscale, reducing resolution to 72 dpi, and deleting unnecessary pages can enable extreme compression.

Q. What is the difference between PDF compression and PDF optimization?

A. Compression primarily reduces file size by lowering image quality, while optimization restructures the file internally for efficiency. Optimization can reduce file size by 10-30% without any quality loss. Applying both together gives the best results.

Important security notes and practical tips for compressing PDFs.

Security considerations:

When using online tools:

- Verify the HTTPS connection (look for the padlock icon in the address bar)

- Do not upload sensitive documents online:

- Contracts, medical records, financial information

- Documents containing personal data

- Confidential business documents

- Check whether the service deletes your files (read their privacy policy)

- Avoid using public computers or public Wi-Fi

Offline tools are required for:

- Legal documents

- Medical records

- Financial statements

- Contracts

- Documents covered by a non-disclosure agreement

Removing metadata:

PDFs can contain invisible information, including:

- Author name

- Creation date and time

- Edit history

- GPS location data (embedded in images)

- Comments (that appear deleted but remain in the file)

Removing metadata in Adobe Acrobat:

1. Tools > Protect > Remove Hidden Information

2. Run the inspection

3. Remove all

Final compression tips:

1. Always back up the original before compressing

You could lose the original while testing multiple versions.

2. Compare multiple tools

The same file can yield different results depending on the tool. Try Smallpdf, iLovePDF, and Adobe, then choose the best outcome.

3. Target 80% of the size limit as your goal

If the limit is 25 MB, aim to compress to 20 MB. Having a buffer prevents problems down the line.

4. Use batch processing

If you need to compress many files, use a tool with batch processing capability.

5. Always verify the file after compression

Open every page and check that text, images, and links are intact.

6. Use descriptive filenames

Save files with names like 'original.pdf', 'compressed_email.pdf', and 'compressed_print.pdf' to keep things organized.

7. Update the PDF version

Older formats like PDF 1.3 and 1.4 are inefficient. Converting to PDF 1.7 or later optimizes the file structure.

Troubleshooting:

- Compression fails: the file may be corrupted - try a different tool

- File size does not decrease: the file may already be compressed or is text-heavy

- Severe quality degradation: reduce the compression level and try again

- File cannot be opened: excessive compression applied - start over from the original