As a general rule, adjoin means that one thing is connected to the other. You can see this from Google's dictionary:
be next to and joined with (a building, room, or piece of land).
You'll notice that they also mention that it should generally be used for a building, room, or piece of land.
However, an attachment is the word you're looking for. This is what it is considered when you put a link in something (attaching a link) or including a document with something (attaching a document). Here's Google's first definition of the term:
They say, later that it could also mean (for your context):
fasten (a related document) to another, or to an email.
To sum up, "adjoin" usually means two physical things, but "attach" would be the correct word in this context.