Native PDF viewer (Documents) for Android email client possible?

[jub]

does this help?

Issue with Whatsapp and pdf file

You can also install the File Browser App from openrepos via Storeman: File Browser | OpenRepos.net — Community Repository System (The version in the Jolla App store is restricted and can’t access the WhatsApp media!), navigate to ~/android_storage/WhatsApp/Media/WhatsApp Documents and open the PDF-file there, which launches the native Document app.