Will reinstalling macOS erase all my photos and documents ?

As mentioned - it should not but do a Time Machine Backup first.

If doing a Reinstall via Recovery Mode and using the Reinstall macOS option - it basically downloads the full installer of the OS associated with the Recovery Volume and installs over the existing version. Thusly, replacing any missing or corrupted System Files.

Not unless a problem occurs. Back them up first.

(204120)