You have downloaded a help file or manual and instead of helping you , it made you search on – “how to open a chm file?” . Well certainly this problem is faced by many and usually we assume that the downloaded file is corrupted or is incomplete. Apparently there can be several other reasons too.

The Table of content (TOC) renders but the content will fail to render. CHM files when installed during program setup will not give you any error. This happens when you  copy a CHM into your computer from other location(network,internet site , etc). So let us go through different troubleshooting steps once forever and check if these work for you, if not, then probably you can blame the downloaded chm file.  Personally I would like to jump to step 4, because it works for 99% of cases.

1) ‘#’ character in filename or dir name.

Please check that the file name or directory name should not have ‘#’ character in their names. If you notice so try renaming it by removing ‘#’ from the name. You must have a simple name for the files and directories in any case (without any sort of special character).

2) CHM and not HLP

If you are on Windows Vista (you probably are), then make sure the file you are trying to open is not .hlp (winHeLP) file. If so, try to convert it using the converter (hh.exe), as Vista does not support .hlp files anymore.

3) Register the CHM viewer

There might be a case when the CHM viewer component is not registered – for any reason. Lets skip the reason and try to register it by typing “regsvr32 hhctrl.ocx” in the command line. If it return OK, then you can try opening your CHM file again. If not then lets move to next step.

4) Security update blocking CHM files

As a matter of fact, CHM files are nothing but compiled html file. Hence this can allow malicious scripts to run if not handled carefully. The windows machines who this updated security fix installed in their machines get these files blocked by default, according to this KB892675. This security patch blocks the HTML files to open when downloaded from remote location. You can resolve this by unblocking the application. TO resolve right click on the chm file and click on properties. Click unblock button in the general tab and click Apply/Ok.  Once you this unblocked, the button will disappear and most likely you will be able to see the content of this chm file.


5) CHM file is in restricted internet zone

When you try to open a CHM file from remote machine, network drive or directly from an unknow site then you might get an error. One way to open this file is to copy it locally and open. However if you want to open it from remote only, then you have to tweak a registry entry. Run regedit, locate HKLM\\SOFTWARE\\Microsoft\\HTMLHelp\\1.x\\ItssRestrictions. If the key does not exists – create it. Add new DWORD subkey named MaxAllowerZone and put there 1. Actually, you can use numbers from 0 to 4 (My Computer, Local Intranet, Trusted Sites, Internet Zone, Restricted Zone). In most cases, Local Internet Zone (1) is enough. If you are not comfortable changing registry or have not done so in the past, please refrain editing and try to concerning an expert. Editing registry if not done properly might land your operating system into trouble.

Hope that saves your day !

Stay Digified !!
Sachin Khosla

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