You are exagerating or possibly mixing PDF with Postscript. PDF allows embedding programs, but AFAIK this is not frequent with normal PDF (as opposed to PDF forms) and AFAIK is not supported by several viewers. PDF allow flexible placement of content inside the file and viewer is supposed to follow internal references to find what it wants to show, but that is mot much different to what happens in other file formats.
Yes, this seem to be normal practice.
What you see depends on fonts in use. If fonts have weird encoding the character codes may look like rubbish. IIUC this some people ply tricks on purpose, to discourage "Web scrapping". This probably can also happen by accident, when creator software mangles embedded fonts to respect copyright restrictions of font creator.
Or as Don Y wrote, this may be due to imperfect OCR.
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