-> Normaly it´s not possible to view a CHM file from a network drive. Please copy the CHM file to a local drive and try it again. If the CHM file is located on your local drive and you can´t open it than check & try the following steps:
-> Your CHM files maybe stored in the folder with a '#' character in the path (for example: C:\PCAN-Basic\C#\'). The hash character signifies an anchor in HTML so the CHM viewer fails to resolve the path properly and to retrieve the content. Please remove the '#' character from the directory name and please also avoid using '?', '&', and '+' characters in directory names.
-> The Microsoft operating system may block access to CHM files to protect your system security. Please run the Windows Explorer, right-click on the CHM file, and select "Properties" from the popup menu. Click on the "Unblock" button below the "Advanced" button on the general page. Click "Apply" to show the content. Once the CHM file has been unblocked, the "Unblock" button disappears.
Note for using CHM files on network drives:
When a CHM file is stored remotely, e.g. on the network drive or on a remote server than you are also unable to display the content of the file. To fix this you must enable a specific security zone.
Modify the ItssRestrictions registry entry to enable a specific security zone.
1. Run 'regedit' command from the command line.
2. Locate and then click the following subkey:HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\HTMLHelp\1.x\ItssRestrictions. >>> Create this registry subkey if it does not exist
3. Right-click the ItssRestrictions subkey, point to New, and then click DWORD Value.
4. Type MaxAllowedZone, and then press ENTER.
5. Right-click the MaxAllowedZone value, and then click Modify.
6. In the Value data box, type a number from 0 and 4, and then click OK.
The values settings are
0 = My Computer
1 = Local Intranet Zone
2 = Trusted sites Zone
3 = Internet Zone
4 = Restricted Sites Zone
7. Quit Registry Editor.
Don´t enable zones about which you are not sure.