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Online PDF Editor Team

How to Compress a PDF (Reduce File Size) — Free, Private, 2026

Reduce your PDF file size in seconds — free and private. Learn how compression works, choose the right level, and fix files that are still too big.

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PDF file being compressed with a visual size reduction indicator

Table of Contents

  1. Why Are PDFs So Large?
  2. How to Compress a PDF Online
  3. How PDF Compression Works
  4. How Much Can You Compress?
  5. Compress Without Losing Quality
  6. What to Do If Your PDF Is Still Too Big
  7. Email Size Limits
  8. FAQ

Why Are PDFs So Large?

You wrote a 3-page letter. It's 12 MB. Why? The short answer: PDFs carry baggage. See our deep dive: Why Is My PDF Too Large? 7 Causes & Fixes. The cliff notes:

Images

A single photo from your phone (12 MP = 4000×3000 px) embedded at full resolution = 5–10 MB alone. Most PDFs don't downsample images on creation.

Fonts

Full font sets embedded (not subsetted) = 100–500 KB per font. A document using 5 fonts = 1–2 MB just for glyphs you might use 10% of.

Metadata & Attachments

Author info, creation timestamps, revision history, embedded files, form data, annotations — all add up.

Embedded Content

Videos, 3D models, audio, JavaScript, hidden layers — rare but massive when present.

No Compression Applied

Many PDF creators (Word, Canva, InDesign, scanner apps) write uncompressed or lightly compressed streams by default.

How to Compress a PDF Online

Step-by-step with Compress PDF

  1. Open onlinepdfeditors.com/compress-pdf
  2. Drag your PDF in (or click to browse)
  3. Choose compression level:
    • Low (Minimal) — Strips metadata, duplicates, optimizes structure. ~10–20% reduction. Visually lossless.
    • Moderate (Recommended) — Downsample images > 150 DPI to 150 DPI, subset fonts, strip metadata. ~40–70% reduction. Screen-quality lossless.
    • High (Maximum) — Downsample images > 72 DPI to 72 DPI, aggressive font subsetting, JPEG recompression at q=50. ~70–90% reduction. Visible quality loss on photos.
  4. Click Compress PDF
  5. Preview the result: file size shown, download starts automatically
  6. Not happy? Try a different level — original file unchanged

Zero upload. WebAssembly runs the optimizer in your browser. Your 100 MB confidential contract never leaves your machine.

How PDF Compression Works

Lossless vs Lossy

Lossless: No visual data discarded. Removes redundancy: duplicate objects, unused metadata, optimizes cross-reference tables, recompresses streams with better algorithms (Flate → LZW/Zlib tuning). Like ZIP for PDF internals.

Lossy: Discards visually redundant data. Downsample images (300 DPI → 150 DPI), JPEG recompression (quality 90 → 50), subset fonts (drop unused glyphs), convert CMYK→RGB for screen. This is where the big savings live.

Downsampling

Images in PDFs often exceed needed resolution. A 300 DPI image on a page displayed at 96 DPI (screen) wastes 90% of its pixels. Downsampling resizes the image to match the target DPI. Our "Moderate" targets 150 DPI — sharp on screen, fine for most printing. "High" targets 72 DPI — screen only.

Font Subsetting

Embedding a full font = every glyph (hundreds). Subsetting = only glyphs actually used in the document. A 500 KB font with 20 characters used → ~20 KB subset. Huge savings, zero visual impact.

Metadata Stripping

XMP metadata, thumbnail images, document info dictionaries, piece info (Illustrator/Photoshop private data), form field appearances — often 100 KB–2 MB of dead weight. Stripped by default in all our levels.

How Much Can You Compress?

Realistic ratios by content type (Moderate level):

{`// Typical Compression Ratios (Moderate Level)

| PDF Content Type           | Original Size | Compressed | Reduction |
|----------------------------|---------------|------------|-----------|
| Text-only (report, contract) | 500 KB       | 150 KB     | ~70%      |
| Mixed text + photos         | 15 MB        | 4 MB       | ~73%      |
| Scanned pages (300 DPI)     | 50 MB        | 8 MB       | ~84%      |
| Presentation (slides)       | 25 MB        | 5 MB       | ~80%      |
| Form with embedded fonts    | 8 MB         | 1.5 MB     | ~81%      |
| High-res portfolio          | 100 MB       | 15 MB      | ~85%      |`}

Low level: 10–20% reduction. High level: add 10–20% more reduction vs Moderate, with visible quality cost.

Compress Without Losing Quality

See our dedicated guide: How to Compress a PDF Without Losing Quality (2026 Guide). The short version:

  • Use Moderate — targets 150 DPI. Screen-perfect. Print-acceptable for most uses.
  • Compare before/after — our tool shows original vs compressed size. Download both, zoom 400%, check text edges and photo detail.
  • Print vs screen — if it's going to a commercial printer, use Low or keep original. For email, web, internal review: Moderate.
  • Re-encode strategy — if Moderate isn't enough: PDF to JPG (300 DPI) → JPG to PDF (Auto page size). This re-rasterizes at controlled quality.

What to Do If Your PDF Is Still Too Big

Re-encode images via PDF→JPG→PDF

Compression works on existing image streams. If the source images are already heavily compressed JPEGs, further JPEG recompression adds artifacts. Better: rasterize pages to fresh JPEGs at your target DPI (PDF to JPG at 150 DPI), then rebuild PDF (JPG to PDF). Clean slate.

Split large files — link to How to Split a PDF & Extract Specific Pages

A 200 MB, 500-page PDF won't compress below ~50 MB. But split into 10 × 50-page files? Each ~5 MB. Email-friendly. Our Split PDF tool handles ranges, burst (every page separate), or even-page splits.

Remove unnecessary pages — link to How to Remove Pages from a PDF

Blank pages, duplicate covers, outdated appendices. Remove PDF Pages deletes them permanently. Extract PDF Pages pulls just what you need.

Email Size Limits

Gmail/Outlook/Office 365: 25 MB attachment limit. Yahoo: 25 MB. iCloud: 20 MB. Proton: 25 MB. Corporate Exchange: often 10–35 MB (admin configured).

If your compressed PDF is still over the limit:

  1. Try High compression (accept quality loss)
  2. Split into multiple emails (Split PDF)
  3. Use a file transfer service (WeTransfer, Firefox Send, your org's SharePoint/OneDrive/Google Drive)
  4. Print to PDF via "Microsoft Print to PDF" — sometimes re-saves smaller (but loses bookmarks, forms, metadata)

See our guide: How to Share Large PDFs by Email (Without Size Limits).

FAQ

How do I compress a PDF for free?

Open onlinepdfeditors.com/compress-pdf. Drag file. Choose level. Download. Free, no limits, no watermark, no account.

Does compressing a PDF reduce quality?

Low/Moderate: virtually no visible loss on screen. High: visible on photos, text stays sharp. See How to Compress a PDF Without Losing Quality.

Why is my PDF so big?

Usually: high-res images, unsubsetted fonts, metadata, no compression applied. See Why Is My PDF Too Large? 7 Causes & Fixes.

What is the best PDF compressor?

For privacy + control + zero cost: our Compress PDF. Client-side = no upload. Adjustable levels. Instant. Free forever.

How do I compress a PDF without losing quality?

Use Moderate level (150 DPI target). Compare before/after at 400% zoom. For print: use Low. See dedicated guide.

How do I compress a PDF to email size?

Compress → if still > 25 MB, split (Split PDF) or re-encode (PDF to JPG → JPG to PDF). See How to Share Large PDFs by Email.

Can I compress a PDF on my phone?

Yes. onlinepdfeditors.com/compress-pdf works in mobile Safari/Chrome. Touch-friendly. Same WebAssembly engine.

How much can I compress a PDF?

Typically 40–85% with Moderate. Text-only: less (already small). Scans: more (huge redundant image data).

Is PDF compression reversible?

No — lossy compression discards data. Always keep the original. Our tool never overwrites your source file.

Does compressing a PDF remove metadata?

Yes — all levels strip XMP, document info, thumbnails, piece info. If you need metadata preserved, use Low level (structure optimization only) or don't compress.

Ready to shrink your PDF? Open Compress PDF — drop your file, pick a level, download. Free. Private. Instant.