Have you ever opened a compiled help file (.chm) and seen an empty viewer, a “The address is not valid” error, or “Navigation to the webpage was canceled”? The file itself is usually fine — the same file often displays without a problem on another machine, or on an older Windows install. The difference is not the file; it’s where the file came from.
When you download a file, Windows attaches a hidden Mark of the Web (an alternate data stream called Zone.Identifier). The CHM viewer renders help content through Internet Explorer components, and for a file flagged this way it refuses to display the content — even after you click through the “Open File – Security Warning” dialog.
The fix: unblock the file
- Right-click the
.chmfile and click Properties. - On the General tab, click Unblock.
- Click OK, then double-click the file to open it.
That’s it — clearing the Mark of the Web lets the viewer render the topics. Only do this for files from a source you trust; the block exists because CHM files can embed arbitrary HTML and script.
Another cause with the same symptoms: if the file lives on a network share or a path containing characters like #, the viewer may fail regardless of the unblock. Copying it to a local folder (e.g. C:\Temp\) rules that out.
CHM has been Windows’ compiled help format since the 1990s, and hh.exe still ships with Windows today — but the format is in maintenance mode, so if you’re authoring new help, modern alternatives (web-based docs) are the better bet — Microsoft itself now recommends moving away from compiled help for new content. For reading an old CHM, though, this unblock trick remains the fix you need.
This worked for me too