The limit you keep hitting
The error always arrives at the worst moment: attachment too large, message not sent. Limits vary by provider but the range is similar everywhere. Many company mailboxes stop at 10 MB, the large providers allow 20 to 25 MB. What matters is not your limit but the recipient's.
There is one detail that surprises people: attachments get re-encoded for sending and grow by roughly a third in the process. A 19 MB PDF can breach a 25 MB limit.
Where the weight comes from
In nearly every oversized PDF, images are the cause. Text weighs almost nothing: a twenty-page contract without illustrations sits at a few hundred kilobytes. Add photos, high-resolution logos or scanned pages and the file grows abruptly.
The most common case is the scan. Scanning at 600 dpi in colour produces several megabytes per page, even when the sheet holds nothing but text.
The direct route
Use Compress PDF to shrink the file in one step. Three levels are available and the choice decides the outcome:
Maximum compression (72 dpi) is meant for screens. Text stays sharp, images visibly coarsen. For a receipt someone will only glance at, that is enough.
The recommended middle ground (150 dpi) fits almost everything sent by email. On screen there is no visible difference, printed it still looks decent.
Minimum compression (300 dpi) is the right pick when the document goes to a print shop or details must stay legible.
When compressing is the wrong answer
Sometimes the file is not too heavy, just too long. A 200-page manual stays large even compressed. Then Split PDF is the better move: the recipient usually needs one chapter, not the whole thing.
If it is about a few pages only, Extract pages is faster. Two hundred pages become three and the size problem solves itself.
Two checks before sending
Is everything still readable? After heavy compression, open the file once, especially with scans in small type.
Is anything in there that should not be? With bank statements and invoices, take the detour via redaction. Important: a black bar only covers, it removes nothing. The text has to be deleted from the file.
In short
Images are almost always the cause, 150 dpi is the sensible default, and with very long documents splitting beats compressing. Everything runs in the browser, so the file never leaves your device.