Thanks for your reply, I only get the option to choose another app and select "always" or "just this once" using "open with", unfortunately, the next time I select the file it does back to the default DropBox app and not the one I selected. Also, the only DropBox app that I have installed is DropBox itself, there is no DropBox PDF viewer app (for example) on my phone, I use Microsoft 365 for PDFs and other documents.