This problem began for me after 11/6. I posted about it here. It does the same thing in Chrome and Safari, on Windows 11 and Mac, with PDFs and PNGs, in my check register and in vendor profiles.

I asked my daughter to try it at work, where they have just recently switched to QBO but have not been attaching files yet. She uses Firefox there, and she reported that the file opened with Adobe Reader and also downloaded.

Does it open in a new browser tab for you or with another program? I suppose that might be a setting in either Firefox or the operating system.

It's obvious to me that this is a bug in QBO, introduced with the November update. Anything attached on or before 11/6 behaves as it should, but files attached after that date want to download.