Chacun à son goût – tastes vary, opinions vary and workflows vary; I think that’s a very good thing. I’ve learned a lot from the approaches other users describe on this forum, and I’ve often adapted them for my own use. 
Perhaps my reluctance to “deface” documents comes from having collected some rare books on topics that interest me. Some of them are hundreds of years old. They have passed through many hands, and probably will after my ownership of them, as well. I try to keep them in good shape, and will never take a highlighter pen to them.
I’ve still got a couple of textbooks from my freshman year in college. At that time I used a pencil to underline important passages. One of those old textbooks has underlining under almost every word in some chapters! Later, I found that my comprehension was improved if instead of mechanically underlining or highlighting material as I read it, I wrote my own notes in a journal book for the course, summarizing important facts and concepts and with page number references to the source. As I got better at that, my notes became shorter, easier to review and more effective in preparation for an exam. The trick is to distill the important stuff from the minutiae, and to only emphasize the minutiae when they illustrate the important stuff.
I was about 13 years old when Vannevar Bush’s article, “As We May Think”, appeared in the Atlantic Monthly. The article excited me then, and I never forgot it. Bush described his dream of a Memex machine, which would allow one to compile a vast collection of information and to add value to that information by adding to it, including finding new relationships among the items in the collection. The Memex machine as he described it was limited to the technologies available at that time, in the 1940s. But his vision translates very well to today’s technologies that include digitized information and the wide availability of digitized material via the Internet.
My DT Pro Office databases are my Memex machine. I think Bush would like those databases, as they allow incorporation of information in many forms, textual, numerical, image, video and audio.
I use rich text notes as they are searchable, can link to all of the various filetypes in my database, can include formatted text, lists, tables, hyperlinks and even embed or link to images, video or audio if desired. Those notes add value (to me) in the databases, and often are assimilated into drafts of a final writing project.
Even when making annotations and notes about a long and rich document, my notes are usually relatively short. If there are many facts or concepts I’ll “branch” a note into multiple documents, with a clear set of links to them, rather like an outlining approach, in which the primary annotation note includes an overview of important topics, each of which might become a separate linked note (and might, in turn, link to other sources or notes).
This approach accommodates my personal foibles and preferences and works for me. It may or may not fit the workflows of others, and there are many and varied approaches that are possible.