For an older application's manual, check whether the contents list appears and whether selecting a topic displays its text. A visible table of contents with a blank topic pane is a different problem from a file that has no working association.
Confirm the file and its source
Check that the filename really ends in .chm. Renaming it to .html will not turn the compiled container into a web page. If you expected a current online manual, ask the software publisher whether it provides one alongside the older Help package.
Keep the publisher's original download and version information. A manual for another software release may open correctly while describing menus your installation does not have.
If double-clicking asks you to choose an app, look for the installed HTML Help program through Open with. On a managed computer, let the administrator handle a missing component or restricted association. Downloading an unrelated “CHM repair” utility is unnecessary for identifying which program should open the format.
Check a downloaded file's blocked status
For a file from a source you have verified and trust, open its Properties in File Explorer and look for an Unblock option. If present, unblocking that individual file can remove the downloaded-file restriction. Close the Help viewer before changing the file's state, then reopen it and select the same topic.
Microsoft's Unblock-File documentation also provides a command-line route and specifically includes a .chm example. For a trusted manual at the illustrative path below:
Unblock-File -LiteralPath 'C:\Manuals\equipment-help.chm'
Replace that path with the actual file. This command removes the file's download zone marker; it does not inspect the contents or establish that the file is safe. Review the source before unblocking, as Microsoft's Unblock-File documentation instructs. Avoid applying it to a whole Downloads folder.
Distinguish restrictions from a damaged package
If your organization permits it, test an authorized local copy rather than a file opened directly from a network location. Preserve the original file. If the same topic stays blank, check another topic and compare with a fresh copy from the publisher.
Record what happens precisely: whether the viewer opens, whether navigation appears, and which topics fail. Those observations give the publisher or administrator something concrete to investigate. Do not assume unblocking will repair missing content or bypass an organizational policy.
Keep the manual's structure when sharing instructions
A CHM package can include a contents tree, an index, and search when its author supplied them. Sending a screenshot of one topic leaves those navigation tools behind. For another Windows reader who needs the complete manual, provide the authorized original package or the publisher's supported documentation route.
If you need to inspect ordinary HTML source, use a plain text editor in that separate workflow. For the compiled manual, finish by opening the exact topic your colleague needs and checking that its version and instructions match the application they are using.