Thanks for the post and welcome to the TestRail discussion forum!
It would be possible to link a file to your test cases or projects. Many text fields within TestRail support Markdown formatting which would allow you to link to a file uploaded to TestRail or located on another server accessible by your team.
For example, inside a project’s list of test cases, you can edit the Description of a section and include a link to a document in the description using Markdown:
You can also use this same method to place links inside text fields of test cases, project descriptions, and other areas in TestRail.
I hope this helps!