Messaging apps cap document attachments, and a phone scan will exceed that cap surprisingly often.
Before sending in a chat, inspect the saved document at a comfortable reading size. Small screen previews can conceal damaged small print. Ask whether the recipient needs searchable text or the original signed version, rather than assuming the smallest file is suitable.
All three presets turn pages into images, removing selectable text and interactive form behaviour. Existing certificate-based signatures are not preserved.
Output size is not guaranteed. There is no exact-KB control or result preview; download and inspect the saved PDF.
Messaging apps usually take large documents, so the real reasons to shrink a PDF are the other person's mobile data and a slow connection. A document that opens quickly on a poor signal is a kindness. Balanced is a good level for this.
If the document is long and only some of it matters, send only those pages with Split PDF. That keeps the text sharp, which is better than crushing every page. Check the compressed copy on your own phone before you send it.
Keep the original or re-export from its authoring app with smaller embedded images. Gentle also rasterises text; Brodidit has no text-preserving PDF compression mode.
A PDF that needs a password to open must be unlocked first with Protect PDF. A very long scanned document can run out of memory on a phone, so try a computer, or split it into parts and compress each one.
Fewer pages whenever the recipient only needs part of the document. It keeps the text selectable and the quality untouched.