Hi sergio.delmaschio, the following article describes how to remove the setting to always use a particular app for a particular type of file. Then the next time you download a PDF, you should get the popup letting you choose the app you prefer.

https://www.lifewire.com/set-and-clear-android-default-apps-3886190

Does that work on yours?

Thank you jscher2000, I had already tried before posting this help request, but I cannot find the defaul built-in Android pdf reader. I cannot find the association made by Firefox with this app either. Thank you again.

I don't know what your PDF reader is called; can you spot its icon in the program list to find its name?

Thank you again jscher2000 for your kind and patient help! There is no special viewer: the downloaded pdf (actually, a php routine, I think, "view_document.php" which downloads the pdf) is opened by Drive. If I change the option, in the Drive settings, for opening supported links, nothing happens, even after rebooting the smartphone: the downloaded files are still opend with Drive, which in turns activates a viewer. This viewer seems to be not a standalone app, it cannot be found in the app list, even looking for apps with "pdf" or "view" or looking for a red icon like the one you can see in the attached images. I could always unisnstall Firefox and then install it again, but it is strange that the association cannot be done in a simple way. Bye, thank you again! sdm

Hi, your second screenshot looks different from my Android 8 so I can only refer to the article I mentioned earlier about where to "Clear Defaults" for Drive.

Ok, thank you!