How to compress a PDF small enough to send
Rewrite the file's structure first — that keeps the text selectable and sometimes it is 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
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.
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.
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.