How to reduce the size of a PDF
Diagnose why a PDF is large, then shrink it the right way for email limits, portals and storage.
PDF is unusual among file formats in that two documents that look identical on screen can differ in size by a factor of a hundred. Understanding why is the key to shrinking one without wrecking it.
Text PDFs versus scan PDFs
A PDF exported from Word, Google Docs, InDesign or a web page contains text as text: font data plus the characters and where to put them. Fifty pages of this is typically under a megabyte. Text is selectable, searchable and infinitely sharp when zoomed.
A PDF from a scanner, a phone scanning app, a fax or a "print to PDF" of an image contains each page as a photograph. Nothing is text; the page is a picture of text. At scanner defaults of 300 to 600 dpi in colour, each page is 2 to 10 MB, and a fifty-page scan can be 200 MB or more.
The quick test: try to select a word with the cursor. If you can, it is a text PDF. If the whole page highlights as one block or nothing selects, it is a scan.
Shrinking a scan-based PDF
This is the case where compression works dramatically. A scan is only as useful as it is legible, and legibility needs far less resolution than scanners default to. Re-rendering each page at 150 dpi and re-encoding it as a JPG at quality 75 cuts a typical scan by 80 to 95% with no practical loss. The PDF compressor does exactly this on your device.
| Scan type | Original | After compression (medium) |
|---|---|---|
| 20 pages, phone scanning app, colour | 45 to 70 MB | 3 to 6 MB |
| 10 pages, flatbed at 600 dpi colour | 60 to 90 MB | 4 to 8 MB |
| 50 pages, office copier at 300 dpi | 40 to 60 MB | 8 to 15 MB |
| 5 pages, receipt photos | 15 to 25 MB | 1 to 2 MB |
For documents that will only ever be read on screen, 100 dpi is enough. For anything that might be printed, stay at 150 or above. Small print, stamps and signatures need 200.
Shrinking a text-based PDF
Here, page-image compression is the wrong tool: it would turn a small, searchable file into a larger, non-searchable one. When a text PDF is unexpectedly large, the cause is almost always one of:
- Oversized embedded images. A 6 MB photo placed in a document stays 6 MB in the PDF even if it is displayed at postage-stamp size. Fix it at source: resize images before inserting them, or use the export dialog's image downsampling option (Word: Compress Pictures; Acrobat: Reduce File Size; InDesign: export preset "Smallest File Size").
- Fully embedded fonts. Some exports embed entire font families rather than the characters used. Choose "subset fonts" in the export dialog.
- Unremoved edit history. Some editors keep old versions of objects inside the file. "Save As" a new copy discards them.
- Vector artwork with thousands of paths. Maps and CAD drawings are large by nature; rasterising just those pages is the fix.
Removing pages you do not need
The simplest size reduction is sending fewer pages. If a portal needs one page of a statement or a single certificate from a bundle, use the split tool to extract just that page. Combine it with compression when the extracted page is itself a heavy scan.
Meeting common size limits
| Destination | Typical limit | Approach |
|---|---|---|
| Email (Gmail, Outlook) | 20 to 25 MB | Medium compression usually suffices |
| Government / university portals | 2 to 10 MB | Medium or strong compression, split unnecessary pages |
| Job application systems | 1 to 5 MB | Export CV from Word directly rather than scanning; compress if scanned |
| Messaging apps | 100 MB to 2 GB | Rarely a problem; compress for the recipient's data plan |
Keeping the original
Compression is one-way. Keep the original high-resolution scan for your records and send the compressed copy. Since every tool on this site runs in your browser, the original never leaves your device in either case.