Most email systems reject attachments somewhere between ten and twenty-five megabytes. Gmail allows twenty-five, Outlook commonly twenty, and corporate mail servers are frequently stricter than either. When a PDF will not send, the useful first step is not to reach for a compressor but to work out why the file is big, because the answer determines which fix will work and which will waste your time.
Divide the file size by the page count. A text document should come in somewhere around fifty kilobytes a page, often much less. A page with a few charts and a logo might reach a couple of hundred kilobytes. A full-page colour scan at 300dpi is two to five megabytes on its own. If your twenty-megabyte file has forty pages of text, something is wrong with the file and structural compression will help enormously. If it has eight pages of scans, the file is exactly the size it should be and no amount of structural cleanup will change that.
This is the case where compression genuinely works, and the cause is almost always accumulated editing history. PDF allows incremental saving: each time an editor saves, it can append changes to the end of the file rather than rewriting it. A document edited over months can carry dozens of generations of itself, with only the last visible. Rewriting the file collapses that to the current state, and reductions of half or more are common. This is also a privacy point worth knowing — an incrementally saved file may still contain text you deleted.
The size is in the images, and the only real lever is fewer pixels or more aggressive image compression. Both reduce quality. The cleanest fix is to scan again at lower settings: 200dpi is entirely adequate for reading text, and greyscale instead of colour roughly halves it again. Between them, that can turn a twenty-megabyte scan into three or four megabytes with no practical loss for a document that is going to be read rather than reproduced. If re-scanning is not possible, you need a tool that downsamples the embedded images, and you should look at the result carefully before sending it.
A report with high-resolution photographs placed in it carries those images at whatever resolution they were inserted. Fixing this properly means resizing the images before placing them and re-exporting the document. Two thousand pixels on the long edge is more than enough for anything that will be read on screen or printed on an office printer.
If the document is genuinely large because it contains a lot of necessary material, splitting it is more honest than degrading it. Send the chapters someone actually needs. A recipient who asked for the appendix does not need the whole submission.
Put the file somewhere and send a link. Every cloud storage service does this free, the recipient gets the full-quality document, and you sidestep attachment limits entirely. The reason to avoid it is confidentiality — a link is a copy on someone else’s infrastructure, and links get forwarded. For an internal document it is usually the right answer. For anything sensitive, shrink the file and attach it.