Rendering a Microsoft Word/Powerpoint document as a PDF is a good thing as then one does not need a Doc/Docx/PPT/pptx viewer anymore while most devices come with a PDF viewer builtin (eg. Chrome :) (and as a bonus it kills the anims if they are there) this while keeping the formatting intact (some minor color changes though depending on one export it).

I tend to keep a whole bunch of things I want to 'read later' in my iBooks collection, just save as PDF and transfer to phone, or if already a PDF just download it directly; zooms great too. I got all kinds of device manuals, but also PADI and other diving reference books; always good to quickly check up on it when in doubt and then to reinforce that information with the knowledge of your dive buddy.

Indeed, for content that does not really need a layout outside of some headers (<h1>) and paragraphs (<p>) HTML is perfectly fine.

Quite a few text portions of conference papers (read: Tex :) can be rendered as markdown and then easily converted to HTML, but it won't feel 'as well', thus PDF is a easier format that also reflects the original intent and format.

IETF RFCs typically can be rendered in a myriad of ways thanks to xml2rfc, then again, one mostly will end up reading them from tools.ietf.org or to keep local, render as PDF and load it into iBooks.