Hi Ethan McGhee,
No worries. Many thanks for sharing the layer. I did have a look at it and it seems that there are no attachments in the layer.
However, what I can see from the attributes is that it is stored as a string in the MapLink field.
Hence what you can ideally do it either attach those PDF as attachments to each of the records so you can view those in the attachment viewer application or if you just want to view that in the popup you can configure using the custom pop up and display the link which can open, which I believe you already have that setup.
Regarding, the attachments there is no easy way out but I could suggest you can either write a script that can help you add attachments to each of the records. I can think of a simple python script that can do the job. Read the MapLink field value, extract the URL for the file and add as an attachment.
Please see this link for more info: Add Attachment—ArcGIS REST API | ArcGIS for Developers
Hope this helps.
Let me know if you have any further queries.
Cheers,
Manish
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Cheers,
Manish