The best program manuals may be hidden in /user/share/doc

Note: My current distro is Linux Mint 19.3 with the Mate Desktop,so the GUI search tool is
mate-search-tool, different desktop, maybe different GUI searcher, but `find` will work in any distro.

Here's something you might want to give a try:

I have the habit that when I manually install a package, typically with synaptic, I will also install the corresponding documentation package, so for say, imagemagick (a powerful image editor, mostly command line) I will also install imagemagick-doc.

From time to time, I will use mate-search-tool, or a command line find command, like:

`find /usr/share/doc -name "index.html" > manuals_list`

the resulting text file will have in it entries like this:

/usr/share/doc/doc-base/doc-base.html/index.html
/usr/share/doc/dvd+rw-tools/index.html
/usr/share/doc/imagemagick-6-common/html/index.html
/usr/share/doc/imagemagick-6-common/html/www/index.html
/usr/share/doc/imagemagick-6-common/html/www/Magick++/index.html
/usr/share/doc/imagemagick-6-common/html/www/api/MagickWand/index.html
/usr/share/doc/imagemagick-6-common/html/www/api/MagickCore/index.html

if I take one of those and put "file://" in front of it, as:

file:///usr/share/doc/imagemagick-6-common/html/index.html ( /// is correct)

I can now copy-paste this into the search line of say, firefox and presto,

I am reading a well written local html manual for that program. I say well written,
because I find that those projects that bother with html documents as part of their
package, are the ones that care about documentation like I do.

Note that this local file address can be bookmarked in your browser
like any other link for later quick reference.