You do not need special software or apps to download PDFs
Almost All operating systems can save PDFs and images
If it’s displayed on the screen are looking at the file is already downloaded on your device
It’s just a matter of saving it locally

Right click or press into the window usually brings up a menu that will contain a way to save it
iOS
Android
macOS
Windows
All have this ability

The offer to download an app to save a pdf is likely the path that would lead to malware as such an app would ask for permissions and then do harm.

Think of it this way:
The programmers who write such apps are proficient enough to know their app is 100% useless to average users so how are they benefitting from writing this app? that app could likely be doing something bad in the background and that’s what the programmers of such a silly app would be benefitting from.
-s