What does DPI mean for your PDF?
On A4, 150 DPI gives an image about 1,240 pixels wide, 300 DPI about 2,480, and 600 DPI about 4,960. Because area grows with the square, doubling the DPI roughly quadruples the file size.
150 is enough for reading on screen, 300 matches what commercial printing expects, and 600 is for archiving or enlarging. Beyond that you are storing detail no page has.
Scanning at a higher DPI than you need is the usual reason a scanned PDF is 80 MB.
What should you actually scan at?
Match the DPI to what the file is for, and nothing higher. Because file size grows with the square of the resolution, the difference between a sensible choice and a careless one is the difference between a 3 MB document and an 80 MB one.
Scanning higher than you need also quietly costs you later: a 600 DPI color scan of a text document is slower to open, slower to OCR, and too big for most upload limits, for detail that no screen and no office printer will ever show.
- 150 DPI — reading on screen, sharing a document nobody will print.
- 300 DPI — the default worth using. Prints cleanly, OCRs accurately, keeps the file reasonable.
- 600 DPI — archiving, fine print, or pages with detail you may need to enlarge.
- Above 600 — only for photographs and artwork, never for text.
Scan in grayscale rather than color for text documents: same legibility, roughly a third of the size.