How to compress a PDF small enough to send
Rewrite the file's structure first, which keeps the text selectable and is sometimes enough. If it is not, rasterising each page makes the file much smaller and turns the text into a picture. Which one is right depends entirely on whether anyone needs to search or copy from the document afterwards.
Compress PDF offers both, and shows the before and after size before you download. Compress PDF
Getting a PDF under a limit without wrecking it
- Find out the actual limit you have to meet, which is usually the upload form's, not WhatsApp's.
- Open the PDF in the compressor and try the mode that keeps the text first.
- Look at the before and after sizes. If that got you under the limit, stop there.
- If it did not, switch to rasterising, and read the warning about what that destroys.
- Check the sample crop of the busiest page at full size before you download, then send the smaller file.
The limits you are actually hitting
WhatsApp will carry a document up to 100 MB, and both that figure and the image limit move. In practice the file that fails is rarely blocked by WhatsApp: it is blocked by a government upload form capped at 2 MB, a university portal at 5 MB, or an email server at 25 MB. Aiming well under any of them is more useful than aiming at exactly one.
There is a second, softer limit nobody prints: the connection of the person receiving it. A 40 MB PDF that WhatsApp accepts cheerfully still has to be downloaded on the other end, possibly on paid mobile data. Sending the smallest file that is still readable is simple courtesy, and it is the version of the file that actually gets opened.
Why the file is that big in the first place
A PDF's size lives almost entirely in its images. A hundred pages of pure text weigh less than one photograph, because text is stored as characters and drawing instructions, a few bytes a letter, while a photograph is stored pixel by pixel. The bloated files people fight with are nearly always scans: a document photographed or scanned at high resolution is a stack of large photographs wearing a PDF's clothes.
That is why knowing your file's nature tells you what will work. A scan responds dramatically to rasterising at a sensible quality. A text document barely responds to anything, because there is almost nothing to squeeze, and it rarely needs squeezing: it was probably small already. And a mixed file, text pages with a few photo pages, is dominated by its photos.
Try the safe mode first
Rewriting the file drops unused objects and repacks its streams. Nothing visible changes and the text stays text. On a PDF that is large because of its structure this can be a big win; on one that is large because of photographs it wins nothing, and a good tool says so rather than reporting a victory it did not have.
What rasterising destroys
Selectable text, copy and paste, search, links, form fields, and everything a screen reader uses. Every page becomes a photograph of itself. For a scanned receipt that loses nothing it had. For a signed contract it loses a great deal, and it cannot be undone from the smaller file.
What a text layer is, and why rasterising removes it
When compression makes the file bigger
Rasterising a page of type can produce a LARGER file, because text stores far more compactly as text than as a picture of text. If a tool hands you a bigger file without mentioning it, that is worth knowing before you send it. This one says so, in red, and tells you to keep the original.
When not to compress at all
A digitally signed PDF is the clear case. The signature certifies the exact bytes of the file; any rewrite, however careful, produces different bytes, and the signature no longer verifies. If a document carries a digital signature and the receiver cares about it, send it as it is or ask the receiver for another route, because no compressor can shrink it and keep the signature valid.
The subtler case is any document where the fine detail is the point: an architectural drawing, a lab result where the smallest print matters, a form someone will zoom into. Compression trades detail for size, and on documents like these the trade runs the wrong way. Ask whether the limit can move before you make the document worse.
Sometimes the answer is a better source, not more compression
If the huge PDF came from scanning, the cheapest fix is often to make it again, smaller: scan at 150 DPI instead of 600, or export the pages at a screen resolution rather than a print one. Compressing a needlessly heavy scan is repairing at the end what was decided at the beginning.
Scanning at a sensible size in the browser
Questions people ask next
Why did compressing barely change my file?
Because the weight was in the images and you used the mode that only rewrites structure. That mode does not touch image data at all, so on a PDF full of photographs or scans there is nothing for it to win.
Will compressing hurt the quality of a scan?
At the default quality, usually not enough to matter for reading. The tool shows a full-size crop of the busiest page at exactly the quality you are about to get, so you can judge it rather than guess.
Is a compressed PDF still a valid PDF?
Yes. Both modes write an ordinary PDF that any reader opens. What changes is what is inside it: after rasterising, each page holds a picture instead of text.
Can it compress a password-protected PDF?
Only with the password. An encrypted PDF cannot be read, and therefore cannot be rewritten, by any tool that does not have the password; that is what the encryption is for. Unlock it first, or accept the size.
What about splitting the PDF instead?
Sometimes the honest answer to a 2 MB limit on a 6 MB file is three uploads, not one worse file. This site does not have a splitting tool, so that is not advice that earns us anything: if the receiver can take the document in parts and the quality matters, splitting beats compressing.