Now on GP 2016 you cannot do this, a warning is popped up stating the following :
The document is in use and could not be attached
TXT, notepad files work fine, but other types such as PDF and word docs will not attach, could be something related to that type of doc too.
Thanks
Comments
What happens as soon as a PDF is clicked, there is a preview that opens. If the SELECT AN ATTACHMENT window has the preview mode turned off the PDF successfully attaches to the Document Attachment Management window without any issues.
Category: System
You can press OK past it and it will attach, or attach when docs are not open. Change in user process, see this forum post
https://community.dynamics.com/gp/f/32/t/241876
Category: System