Like so many other applications, Microsoft’s HTML Help Workshop often chokes on project files whose paths contain spaces.
If you double-click a HTML Help Project File (HHP) in a location whose path contains spaces, the HTML Help Workshop throws an error when you click the Compile button and refuses to compile the CHM file.
Cannot open the file ""C:\Folder Name\File Name.hhp"".
You can work around this issue by removing the quotes from the path shown in the Create a compiled file dialog before you click the Compile button, but you still won’t be able to save any changes you’ve made to the HHP file.
SOLUTION: Instead of double-clicking, open the HHP file from within the Help Workshop via File > Open. This helps the Workshop to handle the paths properly and you will be able to save your changes and compile as expected.