If your topic file names consist of more than one word, use underscores in place of spaces. This enables TOC books and pages to synchronize with topic content displayed in the right-hand pane of the viewer.
You do not have to manually enter underscores between words in place of spaces. To do this automatically, all you have to do is enter the file names and the spaces are replaced with underscores as you type new file names.
From the Tools menu, select Options.
Click the General tab.
Select Use underscores in file names.
The next time you create a topic that uses more than one word in its title/file name, spaces are automatically replaced with underscores.
Tip: If you want to convert all spaces in existing file names to underscores quickly, just go to the Tools menu and click Convert Spaces to Underscores.