My worst habits are largely (TT)RPG manuals. I've got a huge collection from DTRPG especially. Sometimes one RPG purchase will have ten/twenty PDF files inside it (player's guides, GM guides, adventures/modules, etc). (Not to mention the combinatorial explosion if you ever talk yourself into a big sales bundle or two.) There's a bunch of weird reasons to keep them around "read" or not, and make them searchable if I need to look something up or am trying to remember a detail. Syncing some of them to my kindles or to other tablet devices from time to time can be handy for portable retrieval (or on the go skimming). I eventually realized my Books folders were huge and unmanageable even with a detailed folder structure and have been happy with moving that all to Calibre with better tags and metadata management.