iPadOS 26 is all about adding as many unnecessary steps as possible to things that used to be quick and easy. The Preview app is another one of those things. It does exactly what the built-in PDF viewer in Files used to, but in a separate app that slows down workflows. The only benefit I’ve found is that it can act like a PDF accumulator, similar to how Pages and Numbers list all recently used files of those formats in-app.
Best way I found to disable it is to go to a PDF file (or any other file that opens Preview), long hold or two-finger click on the file, go to Open With, and Check the box at the bottom that says “Preview with Quick Look”. This restores the old functionality that iPadOS 18 and older used. You do this once per file type, so if you do this on one PDF file, it works on all PDF files.