What the 25 MB actually covers
It is a message limit, not an attachment limit. The number counts the PDF, the body text, the headers and the encoding overhead, so the file you can attach is always a little smaller than 25 MB. Google states the same figure on its Workspace sharing pages: attachments up to 25 MB go out as attachments, and anything larger is handled differently.
Work and school accounts are the exception. A Google Workspace administrator sets the send and receive caps, and those can sit below 25 MB - some organisations run 10 MB or 20 MB. If your account ends in a company domain and a 15 MB file bounces, the limit is not the one on Google's public page.
You may also see 20 MB quoted for Gmail. Microsoft's support page says 20 MB for both Outlook.com and Gmail, which is where that figure comes from. When the two disagree, trust the service you are sending from.
What Gmail does when you go over
It does not refuse the message. Gmail uploads the file to Google Drive and puts a link in the email instead, which looks like it worked until the recipient tries to open it.
That swap changes three things. The recipient needs permission to open the Drive file, and if they are outside your organisation they may not have it. The document is no longer an attachment, so it cannot be forwarded as a file, archived with the mail, or dropped into a portal that accepts PDFs. And the file now lives in your Drive storage rather than in the mailbox.
On a shared or public computer, that last point matters more than it looks: the link stays live after you sign out, for as long as the file is shared.
Fitting a PDF under 25 MB
Leave a margin. Aim at 20 MB rather than 24 MB, because encoding adds roughly a third: a 20 MB PDF travels as about 27 MB on the wire, which is over the line. A file manager showing 24 MB is telling you the raw size, not the size the mail server sees.
Compress the PDF rather than the mail. Zipping a PDF rarely helps - the file is usually already compressed internally, so a zip saves a few percent at best. Rebuilding the PDF is what moves the number: duplicate objects collapse, streams get packed, and only pages that still overshoot are re-rendered.
That order matters for what survives. A text PDF rebuilt this way keeps selectable, searchable text, which is what you want for a contract, a transcript or a signed form. Pages that are re-rendered become images, so links, form fields and signatures on those pages stop working. If the recipient has to fill anything in, send the original and use a Drive link instead of compressing.
When a link is the better answer
If the file is not a document - a video, a photo set, a design export - compression is the wrong tool. Google's own guidance points at Drive for anything over the limit, and for good reason: a link preserves the original bytes and scales to any size.
Compression earns its place when the file must arrive as a file: a signed contract attached to an application, a scan uploaded to a portal, a transcript a recruiter will download. Those are the cases where a Drive link fails and a smaller PDF succeeds.