In regards to Preview, I still find it insane that it doesn't have an iOS/iPadOS equivalent. Bits of the functionality are scattered all over the place, usually in ways that don't feel as good as they do on the Mac. Sometimes I just want to open a PDF and leave it open, and not have to do it from Files which assumes I want to do something else with it than just looking.
I personally use SumatraPDF on Windows, but it's basically just a fantastic PDF viewer. It does little else in regards to editing/modifying PDFs. Even the PDF viewer in Edge does more.
But for a lightweight, bloat-free experience, SumatraPDF is the way to go.
In regards to Preview, I still find it insane that it doesn't have an iOS/iPadOS equivalent. Bits of the functionality are scattered all over the place, usually in ways that don't feel as good as they do on the Mac. Sometimes I just want to open a PDF and leave it open, and not have to do it from Files which assumes I want to do something else with it than just looking.