I opted for the HTML, show in web viewer (pop-up window, if necessary). And rather than using internet, the files are located in a folder where FileMaker can find them. It allows passing the url based on context (whatever layout you're on).
I built some Help systems in FM within solutions by creating a record for each Help entry, complete with fields for Image, bodyText, heading, Order, and title.
These could be searched or a manual could be printed via a script which found All, sorted them by Heading & Order, and output to Print or PDF.
Some Value Lists for finding by headings and title made it simple to use without printing.
Super easy to update by editing or adding records.