By hard line breaks,
you mean when
breaks are artificially
inserted like this?
I don't remember any of my library PDFs having hard line breaks between sentences in a paragraph, so, well, I'm not sure how to respond to your question. Below I've included some screenshots I took for somebody else once. I don't recall all of the publishers who put out the library PDFs I read, but I think some recent ones were from Allen & Unwin (a lot of them, actually), John Wiley and Sons, and Random House Australia.
A typical reflow looks like this:
Pretty good. Typically my library PDFs would reflow so that I might get two full pages of text, then half a page. Repeat. Left-justified, I usually prefer full justification, but no big deal.
Then there are PDFs which are usually somebody's personal conversions, or at least not typically sold commercially. Such as, er, the occasional torrent I've checked out. Whether reflow looks good or not on those is 50/50.
It's horrible. Absolutely horrible. Luckily I haven't had too much of these, and if I still wanted to read one, I could always zoom. Or just search really desperately for an EPUB version, because I never have any luck at converting PDFs.