Your PDF is a mess. Pages 1-10 are correct. Pages 11-20 are sideways with tops to the right. Pages 21-25 are upside-down. Pages 26-30 are sideways the other way. Pages 31-50 are correct again.
Fixing all four wrong groups in one session is the right move. Flint lets you do it.
Survey the document first
Open rotate PDF pages and scroll through the thumbnail grid. Identify each group of consecutive same-orientation pages. Jot down the ranges: '11-20 sideways right', '21-25 upside-down', '26-30 sideways left'. You're about to fix each as a batch.
Fix one group at a time
Range-select '11-20', apply 90° CCW (because tops were on the right). Range-select '21-25', apply 180°. Range-select '26-30', apply 90° CW. Three operations. Save once at the end. All four groups are now correct.
Sanity check the result
Before saving, scroll through the whole grid. Every page should show upright text. Spot any remaining mistakes? Select just those, rotate to fix, save. The thumbnail grid is your source of truth — if it looks right, the saved file will be right.
Stopping this from happening again
Mixed rotation usually means the source material was scanned inconsistently. If you'll be scanning more of this kind of document, calibrate your scanner: enable auto-orient, feed pages consistently, do a test page before the full run.
FAQ
Can I rotate without selecting first?
Rotation always applies to the selection. 'Select all' is one option; range or click-multiple is another. Nothing's rotated until you click rotate.
What if a page should stay sideways (e.g. a landscape chart)?
Don't include it in the rotation selection. It stays as it was.
Does this work on 1000-page documents?
Yes. Range selection is the same syntax. Saving may take a few seconds longer but it's the same operation.
Can I save snapshots between rotations?
Flint operates on the in-progress version. Save the file, reload it, continue. Two saves is two files.
Group, rotate, save. Fix mixed rotations.