Probably not any form of DRM. It's just that... well... PDF was never intended to be an *editable* file format, it was intended to be a *final*, *immutable* format. Sort of an electronic version of a printed page. You edit your document in Word, InDesign, whatever. When you are happy that is *exactly* what you want the reader to see, you "print" it as a PDF.
Originally, PDF simply did not take *any* steps to make it easy to edit afterwards. Its main concern was to make sure that each letter, dot and picture looks *exactly* how the author intended, in *exactly* the place he intended. It didn't even embrace concepts such as "words" and "paragraphs." It put letters in precise positions on the page. It didn't use whitespace characters such as "space" and "tabs" -- it just moved the letters a bit to the right. Also there are no rule that says that the letters have to be listed inside the PDF in the same order the reader will interpret them. Some softwares make a right mess of that when generating PDFs. Is it intentional, as a sort of primitive DRM to make it hard to extract the text? Maybe, I don't know.
More recent versions of the PDF standard include support for things such as reflowable text -- but that does not necessarily mean that any particular PDF file *uses* those features. So, converting a PDF to an editable format such as Word involves a lot of educated guessing on the part of the software, to figure out which line breaks are also paragraph breaks and such. It's not really different from what an OCR software does to convert an image into text -- it just skips the first step of the process, that is, identifying the individual characters. All the rest -- figuring out words, lines, paragraphs etc. -- is essentially the same.
Regarding punctuation marks... PDFs can also contain a copy of the font you want the reader to see. Nowadays most of those fonts follow the Unicode standard, so the character number for, say, a right-double-angle-quote in a font will be the same as for thousand of other fonts.
But that didn't use to be true. Back in the Dark Ages of the Nineties, there were all sorts of different encodings for characters beyond the (very) basic ASCII set. If you generated your PDF in a Mac with a Mac font, it would look right on a PC because Acrobat Reader would look up the characters in the embedded font... but if you extracted a raw copy of the text, it would look similar to an old Mac txt file opened in Windows Notepad.
An additional complicator (which I have seen mentioned) is
ligatures. Some fonts have special letterforms for things like ff, fi, fl and such, to make the text look better. In old files, again, those were not well standardized, the software pointed to the right positions in the embedded font and it
looked right, but the conversion software may have a hard time figuring out which characters correspond to that particular code point.
To sum it up: all the conversion problem stem from the basic fact that PDFs were not designed to be modified after finalized. It's not deliberate DRM, it's more like... well... the innards of your TV. It is not deliberately designed to make it easy to separate the individual components and assemble a different product. It's just that offering that possibility would make the circuit board bulkier, more expensive and more prone to issues. People want a TV, not a box of electronic Legos.