Document formats are where proprietary lock-in meets daily life. You need Word to properly open a .docx. You need Adobe to do anything useful with a .pdf form. Open standards exist (ODT, ODS) but the world runs on Microsoft's formats whether it likes it or not. The good news: most document formats are more interoperable than their creators would prefer.
The big split is editable versus final. DOCX, XLSX, and PPTX are working documents — you create them, collaborate in them, track changes. PDF is the finished product — layout-locked, device-independent, impossible to casually edit. Everything else is a variation on this theme: ODT for the open-source crowd, RTF for maximum compatibility at the cost of features, TXT for people who don't trust formatting.
E-books sit in this category too, and they have their own compatibility wars. EPUB is the open standard. Amazon spent years insisting on proprietary formats. Kindle finally supports EPUB now, but the scars remain. If you're sharing a document, PDF is the safe choice. If you're collaborating, DOCX is the expected one.
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