Straighten a Crooked Scan

The angle is found by looking for level lines of text: rotate the page until the lines stack up most sharply, and that is the skew. The correction is applied as a rotation of the page content, so text stays selectable — straightening by re-rendering the page would destroy the very text layer that made the angle detectable. Pages that are mostly photographs or diagrams have no text grid to measure, so they are left alone rather than guessed at.

Will my text stay searchable?

Yes. The page is rotated, not re-rendered, so the text layer survives.

Why was one of my pages left alone?

Either it was already straight enough, or it had no clear lines of text to measure.

Can I set the angle myself?

Yes. Switch from automatic to manual and enter it.