PDFs are merely an collection of objects, that can be plainly read by reading the file -- some of those are straight up plain text that doesn't even need to be OCR'd, it can be simply extracted. It is also possible to embed image objects in PDFs, (this is common for scanned files) which might be what you are thinking of. But this is not a design feature of PDF, but rather the output format of a scanner: an image. Editing PDFs is a simple matter of simply editing a file, which you can do plainly as you would any other.