Welcome, @w7hd.
Document Viewer is the Mint PDF viewer. Find it in Menu->Accessories.
If you’re looking at a document in the built-in PDF Viewer extension for Brave, there is a download icon in the top right of the viewer pane you can use to save the PDF, then open it in Document Viewer.
Here’s an image showing the icon location:
If you don’t want to use Brave’s PDF Viewer at all, go to brave://settings/content/pdfDocuments and flip the toggle to Download PDF files instead of automatically opening them in Brave.
Here’s an image showing the Settings location:
Those are both from this somewhat related thread.