When sharing an open PDF file in Firefox Mobile (At least for Android) only the path is shared. E.g. sharing a PDF-File opened from Thunderbird to any message app results in the follwing message:
content://net.thunderbird.android.beta.tempfileprovider/temp/6172b19deeb69d914a9d8d2ad3c8d22328a92941?displayName=xxxxxx.pdf&mime_type=application%2Fpdf
In my opinion it would be better if the PDF-file itself would be shared and can then be easily forwarded.