By some strange logic, the primary and default application for viewing PDFs in Debian is Gimp. If you want to edit the PDF, that might make sense, but that is not the most common use case. There is a bug and discussion about this, but unfortunately, in somebody's stubborn opinion, "it is not a bug", and was closed many years ago.
Luckily it is easy to fix. The default setting can be found in the file /usr/share/applications/mimeinfo.cache which contains this line:
Notice how Gimp is listed first, while the PDF viewers ePDFViewer and Evince are last in the list. You can edit that file (as root). Or if you prefer you can override the user local setting in /home/$USER/.local/share/applications/mimeinfo.cache, and insert something like
The change should take effect immediately, across all applications and browsers, unless the default is overridden there. E.g. Firefox and Chrome have their own internal PDF viewers, however the default MIME applications will be available for selection when the file is downloaded.