Reverse

Back is harder than forward

Going to PDF is a recording. Coming back is a reconstruction, and the difference shows.

Converting a document into PDF throws away the structure deliberately — it keeps what the page looks like and discards what the parts were. Going the other way has to infer the structure again from the appearance, which works well on regular documents and guesses on irregular ones.

The practical consequence: keep the original wherever you can. A round trip through PDF and back is never quite lossless, and the losses are in exactly the places that are tedious to repair.