Almost every problem people have with PDFs comes from treating the format as a single thing. In practice a PDF can be a page description made of vector text and fonts, a container holding one photograph per page, or a mixture of both — and which one you have determines whether a search, a copy-paste, a conversion or a compression pass will work at all.
These guides cover the underlying model: how a PDF stores text and images, how it differs from an editable word-processor file, and how to inspect a file you have been sent before deciding what to do with it.
A PDF is a description of where marks go on a fixed-size page, not a document that reflows — which is exactly why it survived thirty years of format churn.
Use Word while a document is still being written, and PDF once it is finished — the formats are built around opposite assumptions about change.
Try to select a word. If the cursor sweeps across the page without highlighting anything, you have a scan and need OCR before anything else will work.
Modern browsers can parse, render and rewrite PDFs at usable speed, which removes the upload step for most everyday tasks — but not for all of them.