Good question.
Assuming we’re talking about PDFs in the marketing context; beyond using UTMs or HubSpot custom events (enterprise feature) to track the click, visit or download to/of a PDF asset, I suppose you would have to host your PDF content in an external platform to be able to obtain engagement data, and integrate that platform with HubSpot if you wanted contact-level data.
I haven’t found tons of obvious solutions for this but I think Uberflip might do it.
Hope this helps.