I have accomplished this one way using SharePoint and seen another way possible at our local County Assessor's office.
SharePoint Method
Adding a field to the feature layer named something like "RelatedDocumentURL" or something like that will allow you to paste in a url for the SharePoint directory containing the related documents. I would ensure that the link to the directory was modified so that users accessing that only had permission to view documents. Otherwise, they end up getting prompted for a login, which fails. After adding these urls to the field for each feature, a popup can display that field value as a hyperlink in various ways.
If adding individual links to each parcel sounds nearly impossible...
Alternative Method
A custom web application that allows navigation by parcel number should be developed. This requires the documents to be stored in a way that is either retrievable from a database or from a web accessible repository by parcel number. Then, in the popups for the parcel layer, you create an attribute expression that builds a url like: var url = "https://domainname.com/" + $feature.parcel_number. Then, add that attribute expression to popups according to the previously linked documentation.
Neither of these methods is easier. Both require that the documents be indexed somehow. I feel like the Alternative Method puts the work of organizing documents and providing means of displaying them back in the court of the assessors and clerks and allows GIS admins/analysts to provide a means of referencing them instead of doing much of the job of someone in another role.
