I have had the same problem from a long time, and although I agree that if you want to send a PDF doc by using the workaround, it doesnt help for my basic scenario problem.
My problem:
You want to Reply All to a lot of persons (more than a couple, most of the time 20+).
You cant Select All/Copy all of the email addresses, and thats the main problem.
Otherwise I could send any email from a 3rd party app just to send the PDF doc.
So basically you by instinct try 2 methods:
1. Reply All in Mail app and try to insert a PDF doc like any other image or video.
2. Use the 3rd app to send the PDF and paste all of the email addresses.
Problems with 1 (like mentioned on this thread already) is only for Photos/videos in your Camera roll.
Problems with 2, at least for myself that use my companies Exchange email on the Mail app, it is impossible to copy all of the email addresses to Paste on the To: field already open on the 3rd app. It will be great if I could copy All of the email adresses (that most of them are names from exchange itself). Example of the emails on my To: field "John Doe > Jack Hughes > Selena Trish > ..." so on...
So the main problem could be resolved if any of the 2 ways described could be done, either by Select All/Copy "correctly as well" all of the email addresses, or by letting insert a PDF in the Mail app like any photo or video...
The workaround unfortunately doesnt work for this scenario, and I do this a lot (and I need a computer to do so instead)...