Can every PDF reach 100 KB?
Not always. A one or two page scanned document can usually get close to 100 KB at a strong compression level. A long report with many photos, or a PDF that is already text-only and small, will not compress that far. Try the strongest level first and check the result.
If it is still too big
- Lower the compression level once more.
- Split the PDF and upload the pages you actually need.
- Re-scan at 150 DPI instead of 300 DPI — on screen it looks the same and the file is roughly a quarter of the size.
Quality
The selectable text layer is never touched. Only embedded images are re-compressed, so the document stays readable on screen.