I am using 5.717 on Android Pie and cannot open any PDF attachments directly. I am always prompted to choose whether or not to open it with “Antenna opml import” (I’ve never chosen to open any files with this app). There’s the alternative to use 3 other apps (in my case ankidroid, keepass and OsmAnd). Neither of these apps is configurated to be a standard app for anything. The tedious workaround is to save that file and locate it with e.g. TotalCommander, which opens it in Foxit pdf reader.
Is there a way to make K-9 give me the chance to open a file attachment with the app of my choice?