Each page number links into a book. Tell the index where each book actually lives.
A path can be a plain filename (same folder as this page), a relative path
(books/High Guard.pdf), an absolute file:///… URL, or an
https://… URL. Settings are stored in this browser only.
Why you still have to type the folder: browsers never reveal a file's full path to a web page - that is a deliberate security boundary, not something this index can work around. Picking the files gets their names exactly right; the folder is the one thing you set, once.
Chrome and Adobe Acrobat honour #page= and jump to the exact paragraph. Safari
and macOS Preview open the right page but may ignore the paragraph.