I often get multiple PDF files which are not organised in any way, and it would help to be able to preview them. On macOS, I just hit the spacebar when the PDF is selected. Is there anything similar in iOS or iPadOS?
There's Quick Look. Press the document without opening it to get the quick actions menu and select Quick Look. You can also use the share menu in the open PDF to open and save the PDF in the Books app.
There's Quick Look. Press the document without opening it to get the quick actions menu and select Quick Look. You can also use the share menu in the open PDF to open and save the PDF in the Books app.