I have a unique identifier in my list and document library. Is there a way to create a lookup field or something of the sort that link the list item with the corresponding document in the library.
Best approach would be to use some powershell script which will first create a lookup column in list (say A).
Then iterate through items in list A and query list B to for specified value. Add this value to lookup and update item.
Based on the number of items you can do performance upgrades to the script: store IDs of items from list B in cache to avoid querying for the same items twice (or more).
There is already a question/answer how to create lookup fields with powershell: Create Lookup Field Powershell