The web viewers will not honor filepath links, only URLs to https:// paths. Any invalid URL is auto-rewritten to the map's own address.
It works in Pro because Pro is running on your local network. For AGOL to read the file path, the AGOL server would need access to your file directory, which it most certainly does not. If you need the files available via the web, consider either using attachments on the features, or else storing the plans in a location that can be exposed to the web.
- Josh Carlson
Kendall County GIS