Open Clean a scan from the Home screen (or use its own page link).
Tap Choose files and select all the files you need (you can select several at once).
Choose Black & white.
Tap the big purple Start button and wait a few seconds.
Check the result line, it shows the old and new size (for example 4 KB → 3 KB). If the text looks blurry, go back and choose a bigger size.
When you see Done!, tap Download. The new file is saved on your phone or computer.