HI Nicholas,
There are a few different ways to accomplish this, depending on where your excel file is located and how it needs to be referenced by your data. If you have the same excel document for all, then you can add the link during the pop up configuration process. If you have a different excel document for each feature, you are going to want to add a string field to your data set and add the url to your excel document as a field. If you include the url field in a pop up it will be automatically hyperlinked.
If the link is accessible on the internet, then adding the url using the above mentioned workflow is straight forward. If you have a document locally, then you will need to add your file to a local web server as described in this post https://community.esri.com/message/489020?sr=search&searchId=346230c2-a246-4c42-9415-6dfa4bd58175&se.... Remember that if your file is local only users on your network who have access to the document will be able to access it.
-Kelly
