Page-level operations are the most common PDF tasks and also the safest: copying pages between documents does not re-encode anything, so the text stays selectable and print quality is unchanged. What goes wrong is usually ordering, orientation and mixed page sizes rather than quality.
These guides explain how page copying works, how to choose between splitting, extracting and removing, and how to keep a long document coherent when you cut it apart and put it back together.
Merging copies pages between documents without re-encoding them, so quality never changes — the things that go wrong are order, orientation and mixed page sizes.
Split when you need many files, extract when you need one smaller file, and remove when you want the original minus a few pages.
Capture consistently, fix orientation before merging, then compress and OCR once at the end rather than per file.