Where a 100KB limit comes up
Many government, exam, visa and job application portals accept only 100 KB per supporting document: a copy of an ID, a diploma, a signed one page form. A single page scanned with a phone is often 1 to 5 MB, so it has to lose most of its weight before the form will take it.
The target is already set to 100KB. If the form asks for something else, type the value in KB. Typing a little less, for example 95, leaves a margin for portals that count sizes differently.
What to expect at 100KB
A digital PDF of one or two pages, such as a form saved from Word, usually fits with its text still sharp and selectable. Only the images inside are recompressed.
Scans are harder. If shrinking the images is not enough, the pages are turned into images at a lower resolution, from 110 down to 60 DPI. The text stays readable on screen but can no longer be selected, and the result row tells you when this happened. Long documents rarely fit in 100 KB at all; if the portal allows several files, split the PDF with Split PDF and upload the parts.
Scanning with a small file in mind
Scan in grayscale instead of color and at 150 to 200 DPI. A grayscale scan of a printed page is a fraction of the size of a color one and stays much clearer after compression. Remove blank backs of pages with Delete PDF pages before you compress.