The File Reader web part is added to the /SitePages/CTFiles/File-Reader.aspx page along with the Tracker web part. The page is used to open documents in the web viewer for tracking.
All documents and files to be tracked must be opened using the File Reader page. This is the native page used by My Reads.
Any files viewed without using the File Reader page will not be tracked.
This is especially true for Hidden Tracker content. Unless it’s displayed in My Reads or using the PnP Modern Search web part, tracking will not take place.