Only URLs starting with "http" or "https" can be registered as "Outside Links".
URLs starting with "file" or Windows file path formats cannot be registered.
Examples of unregisterable links:
- C:\Users\username\Documents\foldername\filename
- file://123.456.7.890/foldername/filename
Related:
Can I attach files such as Excel, PowerPoint, or PDF to manuals?
https://help.teachme.jp/hc/en-us/articles/900006444983
Add link to manual (Web Browser)
https://teachme.jp/8/manuals/8733762