Readly uses two techniques to fix this:

RSVP reading Shows one word at a time on a clean black screen. This removes eye movement completely and keeps your reading pace steady.

Inline spotlight reading Adds a moving highlight bubble directly on the PDF. It guides your eyes word by word, like a focused magnifier, without breaking the document flow.

Other features - Clean focus mode with no distractions - Offline library and resume progress - Custom highlight colors and themes - Quick jumps across long PDFs

Play Store: https://play.google.com/store/apps/details?id=com.bufferlabs...

Feedback is welcome.