Scanners and phone cameras record orientation differently from how the image is stored. A landscape photo of a portrait page often ends up rotated 90 degrees in the resulting PDF.
Steps
Open Rotate PDF and upload the file
Choose the rotation direction that fixes the page - 90 degrees clockwise fixes most sideways scans
Apply and download the corrected PDF
Better results next time
When scanning with a phone, hold the device in the same orientation as the page
The Files Office phone scanner corrects orientation during capture - scanned pages rarely need rotating afterwards
If only some pages are wrong, extract them, rotate, and merge the file back together with Merge PDF