A light touch is usually enough
Because 5 MB is a relatively generous limit, start with the Recommended level. It keeps photos and diagrams looking sharp while trimming the extra weight that design tools and phone scanners add.
Very large file (30 MB+)?
- Use the Strong level - it reduces large embedded images the most.
- Check for pages scanned in colour that could be greyscale.
- If the portal accepts several files, split the PDF into parts.
Why 5 MB is a good size for email too
Most email services accept attachments of 20-25 MB, but large attachments are slow to send on mobile data, can bounce from smaller company mailboxes, and fill up the recipient's inbox. Keeping a report, portfolio or scanned pack under 5 MB makes it quick to send, download and open on a phone.
Check the important details
With a 5 MB target there is room to keep quality high, so after compressing, zoom in on photos, charts and small print. If they look as good as the original, you are done - there is no benefit in compressing harder than the limit requires.