Some PDFs do not save/print properly. They are displayed correctly, but that is all. For these problematic PDFs, Share > Print does not do anything, i.e. no print dialog box or error message. Share > Save to files does appear to work, but the result is an empty PDF file, typically 4-7 KB. I think the affected files are those generated for me, eg bank statements and receipts. The problem has existed for a long time on multiple new iPhones and iPads with up-to-date Firefox iOS/iPadOS (for example iPhone 15 Pro with iOS 17.6.1). On my MacBook Air it all works fine.