An 18 MB PDF rejected by a form capped at 5 MB, an email bouncing back, a ten-page scan that takes a minute to open on a phone: sooner or later a PDF's weight becomes a problem. The good news is that you can almost always slim it down a lot, as long as you understand where the weight comes from and do not apply the same remedy to a text report and to a scan.
Where a PDF's weight comes from
A PDF is a container. Inside you find text (very light: a few dozen kilobytes for ten pages), embedded fonts (20 to 300 KB each, sometimes more for a full Asian font), vector graphics and, above all, images. In nine cases out of ten, the images are the weight.
Take an A4 page scanned in colour at 300 dots per inch: it measures 2,480 × 3,508 pixels, about 8.7 million pixels. Stored uncompressed, that is 26 MB per page (3 bytes per pixel). Even JPEG-compressed, such a page commonly weighs between 0.5 and 2 MB. A ten-page scan quickly passes 10 MB, whereas the same content typed in a word processor would weigh under 200 KB.
That difference in nature is why there is no single setting: you do not slim down a scan the way you slim down a document exported from Word.
The three levels of compression
The Compress PDF tool offers three levels, described without embellishment:
| Level | What changes | Text still selectable? | Pick it when |
|---|---|---|---|
| Light | The file is cleanly rewritten (unused objects and duplicates removed). Images are untouched. | Yes | The PDF comes from software that exported it badly, and you tolerate no loss. |
| Balanced | Embedded photos are brought down to 150 ppi by default and re-encoded as JPEG. | Yes | The general case: scans, illustrated reports, presentations. |
| Maximum | Every page becomes an image. | No | A last resort, when a size limit is mandatory and the text does not need to be copied. |
The real saving depends on the content. A PDF full of photos or scans can lose 60 to 90% of its weight at Balanced. A pure-text PDF has little to lose: Maximum would mostly “slim” it by turning it into images, which costs text search and can even make it heavier.
Compressing step by step
- Open Compress PDF and drop your file (80 MB maximum per file; several files at once are possible, returned in a ZIP archive).
- Leave the level on “Balanced”, unless you know the file is already pure text.
- Click “Compress” and read the three figures shown: size before, size after, percentage saved. They are measured on the produced file, not estimated.
- Look at the first-page preview, then open the downloaded PDF and flip through it before sending it: check pages with photos or stamps in particular.
- If the file is still too heavy, switch to “Maximum” or lower the image resolution (120 ppi is enough on screen).
If the tool cannot produce a smaller file, it hands you the original and tells you so: a PDF can never come out heavier than it went in.
Practical tips for better results
- Compress from the original. Recompressing an already compressed PDF degrades the images again for little gain.
- Shrink images before inserting them. If you build the document yourself, photos resized to their display size avoid the problem at the source (see Resize and crop an image for the web, Instagram or YouTube).
- Scan at 150 or 200 ppi for an everyday document. At 300 ppi you only gain legibility for very small type; mostly you multiply the weight: going from 300 to 150 ppi divides the pixel count by four.
- Scan in greyscale or black and white documents with no useful colour: the weight drops with no loss of legibility.
- Keep the original. Image compression is irreversible.
Common mistakes
- Picking “Maximum” by reflex, then finding that the text can no longer be selected and Ctrl+F finds nothing.
- Trying to compress a password-protected PDF: unlock it first with Unlock PDF if you are entitled to.
- Judging the result by size alone and not by appearance: always open the produced file.
- Compressing three times in a row hoping to gain each time: the second pass almost always yields very little.
Honest limits
- Processing happens in your browser's memory, hence the 80 MB limit per file. A very large PDF can fail on an entry-level phone: try on a computer.
- Images the tool cannot process safely (transparency, masks, indexed or CMYK colours) are left intact: a PDF full of them will shrink less.
- Digital signatures are invalidated by any change to the file, compression included: do not compress a document that is already electronically signed if the signature matters.
- No compression makes a blurry scan sharper: if the text is unreadable before, it will remain so.
Frequently asked questions
Is my file uploaded to a server?
No. Compression is done by your browser; nothing is transmitted or stored. That is also the reason for the 80 MB limit per file.
Why does my text PDF barely shrink?
Because it already weighs little: text and fonts take a few hundred kilobytes at most. Only images offer real room to manoeuvre.
Which resolution should I pick for a PDF read on screen?
150 ppi is enough for a document read on screen or sent by email; keep 200 to 300 ppi if the document will be printed or the type is very small.
Does the text stay selectable after compression?
At Light and Balanced, yes. At Maximum every page becomes an image: the text is no longer selectable, unless you then run the file through OCR PDF.
Can I compress several PDFs at once?
Yes: drop them together, they are processed one after the other and you get everything back in a ZIP archive.