Thank you for submitting your idea. We believe that this already exists. By clicking Add attachments, you can link to project documents. You can also create links from the plan set by marking up the plan sheet and creating a form from there.
Once linked, the related document is shown right above the related forms.
With that said, we are always looking to improve and believe that there is an opportunity here to make it easier to link forms to plan sheets. We will take that into consideration for a future release.