PDFs are not real ebooks, they are electronic documents intended to be WYSIWG preview for paper. PDFs define a physical page size, so most won't be usable on a screen not large enough. A PDF can contain postscript, OCR text, scanned images of books, vector graphics and other images as well as forms, either layered or on separate pages.
Kindle (Amazon) can mean:
1. An eink based ereader, hardware to read Kindle ebooks, though they will show PDFs if the PDF page is small enough. The first model was in 2007.
2. An App for PC, Mac, iOS or Android.
3. An ebook format (though some Kindle formats are fixed layout or encapsulated PDF, so won't work on ereader hardware). There are several formats possible in a mobi or azw file, including KF7, dual KF7+KF8 and others. An azw3 file is KF8. Most have DRM. KFX format can use several files and always has DRM. Kindle ereaders and apps might also read old Mobipocket prc files. The Kindle formats were created (maybe 2007) after Amazon bought Mobipocket (2005).
4. KDP (Kindle Direct Publishing) is the service and interface to upload ebooks for sale. The epub format is best.
5. Send to Kindle. A Cloud service where you send an epub to Amazon and they keep a copy and also send it via WiFi or Mobile to the Kindle Hardware. Sideloading (copy by USB) of a Kindle
Amazon also has a Fire Tablet, it's Amazon's version of Android linked to the Amazon store instead of Google Playstore. It was confusingly called a Kindle Fire for a while. It has a pre-installed Kindle app that's not quite the same as the Android or iOS apps.
The Kindle ebook ereader will not read all Kindle "ebooks", because some only work on Amazon's Web app, some only work on the Fire and some only work on the Apps. However the Kindle ereader is the simplest way to read real Kindle ebooks.
The Kindle ereader never has been leading edge. Sony was once more advanced and now Kobo is superior for ease of use. It's possible to read Kindle ebooks on other ereader hardware, but they need converted to epub, and often need DRM removed. Fixed layout Kindle books won't convert, sometimes you can extract a PDF.
See Calibre Forum here.