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
(Drinax/Jayne's Guide.pdf), an absolute file:///… URL, or an
https://… URL. Settings are stored in this browser only.
Books in more than one folder? Use Choose the folder… and pick the
one they all sit under. The browser reports each PDF's position within that folder, so
Core/ and Drinax/ are filled in for you and the paths above end up different
from each other. You give the root once; the rest is discovered.
Why you type the folder at all: browsers never reveal a file's full path to a web page - a deliberate security boundary, not something this index can work around. Picking gets the names and the subfolders exactly right; the root is the one thing you set, once. If this page already sits in that root, leave the box blank.
A book somewhere else entirely - another drive, a NAS, a web host - does not need to move.
Type its own file:///… or https://… URL into its row and
Apply folder will leave that row alone.
A link opens the book at the right page (via #page=), at whatever zoom your
PDF viewer is set to. It does not scroll to the exact paragraph — browser PDF viewers
handle that inconsistently, so the index aims the page and lets you read from the top.