Best PDF Compression Techniques Compared

PDF compression is not a one-size-fits-all process. Different techniques produce different results in terms of file size reduction and quality preservation. Understanding the available compression techniques and their trade-offs helps you choose the right approach for each document. This guide compares the best PDF compression techniques available in 2026.

Understanding PDF Compression

PDF compression involves several distinct techniques, each targeting different types of data within the file:

Image Compression

Images are typically the largest component of a PDF. Compression techniques for images include:

• Downsampling: Reducing the resolution (DPI) of images. A 600 DPI image downsamples to 150 DPI with no visible difference on screen.
• Recompression: Converting images to more efficient formats (e.g., from uncompressed to JPEG).
• Quality reduction: Increasing JPEG compression, which reduces both size and quality.
• Color reduction: Converting 24-bit color to 8-bit or grayscale.

Font Optimization

• Font subsetting: Embedding only the characters used in the document rather than entire font families.
• Font unembedding: Removing embedded fonts that are commonly available on most systems (risky if the document will be viewed on diverse systems).

Object Cleanup

• Removing unused objects: Deleting objects that are referenced nowhere in the document.
• Removing duplicate objects: Eliminating objects that appear multiple times.
• Flattening layers: Merging visible and hidden layers into a single layer.
• Removing metadata: Stripping unnecessary metadata, thumbnails, and document properties.

Stream Compression

• Flate compression: Applying zlib compression to content streams (text and vector graphics).
• Recompressing streams: Applying more efficient compression to streams that were stored uncompressed or with older algorithms.

Technique 1: Downsampling Images

Downsampling is the most effective compression technique for image-heavy PDFs. It reduces the resolution of embedded images to a target DPI.

How It Works

If a PDF contains a 600 DPI image that is displayed at 2 inches wide, the actual pixel data is 1200 pixels wide. If the document is only viewed on screen (where 150 DPI is sufficient), the image only needs 300 pixels. Downsampling from 600 to 150 DPI reduces the image data by 93%.

Choosing the Target DPI

| Target DPI | Quality | File Size Reduction | Best For |

|---|---|---|---|

| 72 | Low | Very high | Web thumbnails |

| 150 | Good | High | Screen viewing |

| 200 | Good | Medium | High-quality screen viewing |

| 300 | High | Low | Printing |

| 600 | Very high | Minimal | Archival, fine detail |

Downsampling with Ghostscript

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4    -dDownsampleColorImages=true -dColorImageResolution=150    -dDownsampleGrayImages=true -dGrayImageResolution=150    -dDownsampleMonoImages=true -dMonoImageResolution=300    -sOUTPUTFILE=compressed.pdf input.pdf

Downsampling with Adobe Acrobat Pro

In the PDF Optimizer (File > Save as Other > Optimized PDF):

1. Go to Images.
2. Set "Downsample" for color, grayscale, and monochrome images.
3. Set the target resolution for each type.
4. Choose compression (JPEG for photos, ZIP for flat color).

Technique 2: JPEG Recompression

Converting images within the PDF to JPEG format with appropriate quality settings can dramatically reduce file size.

How It Works

JPEG compression is designed for photographs and continuous-tone images. It achieves high compression ratios by discarding data that is less noticeable to the human eye. The quality setting (1–100) controls the trade-off between size and quality.

Choosing JPEG Quality

| Quality | File Size | Visual Quality | Best For |

|---|---|---|---|

| 50 | Very small | Noticeable artifacts | Thumbnails |

| 60–70 | Small | Some artifacts | Web display |

| 80–85 | Medium | Good | Most uses |

| 90–95 | Large | Very good | Print, archival |

| 100 | Very large | Lossless | Not recommended |

JPEG vs. JPEG2000

JPEG2000 offers better compression at the same quality but has limited support in older PDF viewers. For maximum compatibility, use standard JPEG.

JPEG Recompression with Ghostscript

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4    -dColorImageFilter=/DCTEncode -dAutoFilterColorImages=false    -dColorImageDownsampleThreshold=1.5    -dDownsampleColorImages=true -dColorImageResolution=150    -sOUTPUTFILE=compressed.pdf input.pdf

Technique 3: Font Subsetting

Font subsetting embeds only the characters actually used in the document, rather than the entire font.

How It Works

A full font file can be 100KB–1MB. If your document uses only 100 unique characters from that font, subsetting reduces the embedded data to just those characters — often 10–20KB.

Font Subsetting with Ghostscript

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4    -dSubsetFonts=true -dEmbedAllFonts=true    -sOUTPUTFILE=compressed.pdf input.pdf

Font Subsetting with Adobe Acrobat Pro

In the PDF Optimizer, go to Fonts:

1. Check "Embed only used characters (subset)."
2. Set the subset threshold (e.g., embed as subset if less than 100% of characters are used).
3. Unembed fonts that are commonly available if you do not need exact font reproduction.

Technique 4: Object Cleanup

Removing unnecessary objects reduces file size without affecting visible content.

What to Remove

• Unused objects: Objects in the file that are not referenced by any page.
• Embedded thumbnails: Page thumbnails that are no longer needed (modern viewers generate their own).
• Document metadata: Excessive XML metadata, custom properties.
• Hidden layers: Layers that are not visible.
• Form field redundancies: Duplicate or unused form fields.
• Annotations: Author information, revision history, and other metadata from annotations.

Object Cleanup with Ghostscript

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4    -dDetectDuplicateImages=true -dCompressFonts=true    -sOUTPUTFILE=cleaned.pdf input.pdf

The `-dDetectDuplicateImages` flag identifies and deduplicates images that appear multiple times.

Object Cleanup with qpdf

qpdf --linearize --object-streams=generate input.pdf output.pdf

Linearization optimizes the PDF for web viewing and can reduce size. Object streams compress objects more efficiently.

Technique 5: Stream Compression

Content streams (text and vector graphics) can be compressed with Flate (zlib) compression.

How It Works

PDF content streams contain the instructions for rendering pages. These are text-based and compress well with zlib. Most modern PDFs already use Flate compression, but older PDFs may not.

Stream Compression with Ghostscript

Ghostscript applies Flate compression by default when writing PDFs:

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4    -sOUTPUTFILE=compressed.pdf input.pdf

Comparing the Techniques

Effectiveness by Document Type

| Technique | Text-Heavy | Image-Heavy | Scanned | Mixed |

|---|---|---|---|---|

| Downsampling | Minimal | Very high | Very high | High |

| JPEG recompression | Minimal | High | Very high | Medium |

| Font subsetting | High | Minimal | Minimal | Medium |

| Object cleanup | Low | Low | Low | Low |

| Stream compression | Medium | Low | Low | Medium |

Combining Techniques

For maximum compression, combine multiple techniques. The order matters:

1. Downsample images first (largest impact).
2. Recompress images with JPEG.
3. Subset fonts.
4. Clean up objects.
5. Compress streams.

All-in-One Compression with Ghostscript

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4    -dPDFSETTINGS=/ebook    -dSubsetFonts=true -dEmbedAllFonts=true    -dDetectDuplicateImages=true -dCompressFonts=true    -dDownsampleColorImages=true -dColorImageResolution=150    -dDownsampleGrayImages=true -dGrayImageResolution=150    -dDownsampleMonoImages=true -dMonoImageResolution=300    -dColorImageFilter=/DCTEncode -dAutoFilterColorImages=false    -sOUTPUTFILE=compressed.pdf input.pdf

This command applies all major compression techniques in one pass.

Quality Verification

After compression, always verify:

1. Text legibility: Zoom to 100% and check text crispness.
2. Image quality: View images at 100% and 200% zoom.
3. Color accuracy: Compare colors to the original.
4. Interactive elements: Check links, bookmarks, and form fields.
5. File size: Confirm the reduction meets your needs.

Conclusion

The best PDF compression technique depends on your document type and quality requirements. For image-heavy and scanned PDFs, downsampling and JPEG recompression are the most effective. For text-heavy PDFs, font subsetting and stream compression provide the biggest gains. Combining multiple techniques produces the best results. Always verify quality after compression and adjust settings if the quality is not acceptable.