Hi, I'm John! I will help you with this.
It doesn't directly support attaching PDF files within the Phone Link app itself. If you need to share a PDF, you could try sending it through OneDrive on your PC and OneDrive mobile app on your phone.
To provide feedback to Microsoft, please use the Feedback Hub by pressing Windows key + F key to your keyboard.
"Send feedback to Microsoft with the Feedback Hub app"
Should you need further information please let me know.
Click "Add comment" to let me know.
Regards,
John J.D.