What actually fits in 1 MB
It depends almost entirely on whether your pages are scans or text. A text-based PDF — something exported from Word, or a generated report — is often under 1 MB already, and if it is not, it will get there easily. Scanned pages are the hard case, because each page is a photograph.
As a rough guide, five to ten scanned pages will reach 1 MB and still be comfortably readable. Twenty pages will get there but look soft. Fifty scanned pages will not fit into 1 MB in any form you would want to send to anyone — at that point you are better off splitting the document or asking whether the limit really applies.
Where the 1 MB limit comes from
It is the default attachment ceiling on a lot of older web forms, and it shows up constantly in job application portals, university submission systems and government e-filing. The number is usually inherited from a default setting rather than chosen, which is why it feels arbitrary.
If it will not reach 1 MB
The tool tries progressively lower image quality, and when that runs out it reduces the resolution and tries again. If it still reports that it could not reach the target, the document has more pages than will fit at any readable quality. Splitting it into two files is usually the better answer, and most portals that cap at 1 MB accept multiple attachments.
Will the text still be selectable?
No. Compression here works by redrawing each page as an image, so text in the result cannot be selected, searched or read by a screen reader. For a scanned form or an ID document that costs you nothing. For a contract someone needs to search, use a gentler target or a different approach — the Acrobat method keeps text intact.