Ever wondered why your .CHM file(s) does not work?


I had couple
of .CHM files, specially for C# .NET related stuff and I saw some of the chm
files when I open them I see "This page cannot be displayed" error.
Finally I figured out that there could be two reasons for this:

[Option 1
courtesy chan.chawla from the posted comment, adding as the first option to be
tried out before going any further]

1. Double
click the .chm file. It will open a security dialog. Uncheck "Always ask
after opening this file". Alternatively, right click on the file you want
to open, click Properties and then click Unblock. This should do the trick.

2. If you chm
file name or the path contains '#' character, just get rid of that and I guess
you would be able to start viewing your chm files again.

3. If nothing
works, try this.

Because of a
Microsoft Service Pack, the chm files does not have enough permissions (by
default). This case is usually when you try to open .chm files over network.
The resolution for this is as follows:

  • Click Start, click Run, type regedit, and
    then click OK.
  • Locate and then click the following
    subkey: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\HTMLHelp\1.x\ItssRestrictions
    [If this registry subkey does not exist, create it]
  • Right-click the ItssRestrictions subkey,
    point to New, and then click DWORD Value.
  • Type MaxAllowedZone, and then press Enter.
  • Right-click the MaxAllowedZone value, and
    then click Modify.
  • In the Value data box, type 1, and then
    click OK.
  • Quit Registry Editor.

more information check http://support.microsoft.com/kb/896358.

4. One more reason provided for unable to open chm files [courtesy colin.cole]

Any chm file that is in a folder named *.col* will not open.
Example paths that will cause this error:
c:\documents and settings\john.coltrane\desktop\TFSInstall.chm

Solution: Move file somewhere else on your computer.

Comments (46)

Cancel reply

  1. chan.chawla says:

    This is what actually helped me!!


    After downloading the document, open it from the location where it was saved and a security warning will appear. In order to view the help documents properly, the checkmark must be cleared from the ‘Always ask before opening this file’ check box. The files will then open properly from the PC. Alternatively, right-click on the file you want to open, click Properties, and then click Unblock.


  2. Baruch says:

    I don’t believe it but I had my C# file in a older called C# and it didn’t like the "#".  I changed it to csharp and it worked.  I don’t believe it.

  3. rakesh juyal says:

    Cool, my .chm file was inside a directory with name containing ‘#’. I removed ‘#’ ,and now i can view .chm files.

  4. Manish Haria says:

    I didn’t have the subkey ItssRestrictions in my registry, but just changing the # sign in the folder containing the .chm file fixed the issue (just like Rakesh Juyal menioned above)!

  5. MrMeek says:

    OMG, i just had the same issue today. Taking the #s out of the directory name fixed all my C# chms! Thanks~!

  6. raghunand says:

    That was very very very helpful!!! Thanks a lot!!!

  7. Pete Maher says:

    ….and watch out for dots in folder names too.  My chm was on a network folder which I was referencing via a UNC path using the ip address.

  8. Pete Maher says:

    …or maybe it is just a problem with any network share – moving the file locally meets with success.  But moving it to a UNC share using a server name rather than an IP address still had the problem.

    (on XP).

  9. kishore says:

    It was very Help full.

    Thanks a lot 🙂

  10. Marc says:

    A very helpul hint, thanks a lot!

  11. Rathna says:

    It was really very helpful.Thanks a lot:-)

  12. salil says:

    thanks man..

    i removed the # from my file name and path and now i am able to view it.

  13. axea says:

    hey neeraj!

    That was very useful.Thank you so much for posting this – very useful piece of information.

    best wishes

  14. Gizew says:

    Tnxs, I had the same problem and the problem was C# in my folder name.

    After I renamed C# to C-sharp, everything was okay,

    Thank you once again for this info.

  15. lome says:

    Thanks Neeraj. It is very helpful.

  16. gopalmyneni says:

    Double click the .chm file. It will open a security dialog. Uncheck "Always ask after opening this file". Alternatively, right click on the file you want to open, click Properties and then click Unblock.

    —-   there  is no unblock

    —-Uncheck "Always ask after opening this file"

       there is no such check box for me…

     post not useful

  17. NeerajAg says:

    Hi gopalmyeni,

    Can you send me some more details about the problem. Did you checked if there is any # character in the file or entire path?

    Also what error do you get when you open your chm file?



  18. Carol says:

    Wow, open from the folder where the file is located and just uncheck the box that asks about opening the file, and it opens! Much nicer solution than having to go to the command line and/or edit files.


  19. aarti says:

    Hey, Thanks ..

    I created a registry and it really worked. 🙂

  20. Martin says:

    Great! Now I can open chm files…I was wondering what on earth was the problem, now it works.

  21. Nehal says:

    Thanks mr Chan Chawla, I alwayse wonder y my chm file is not working on my computer, and u have a simple solution that i dont know. Thanks again.

  22. Sumit says:

    It works thanks.

    Nice info as i have been wasting bandwidth downloading the same file again and again from different locations to see that it does not work.

  23. Prem says:

    Hey Neeraj, thanks for this post. Unblocking the file property simply did the trick.

  24. lee says:

    I still can not open this file since i have the

    hh.exe and also set the open way and link path.

    It just still can not open.

  25. Neeraj says:

    Hi Lee,

    I am surprised if all the above options are saturated. If the document is not very confidential, can you share that with me? Also let me know which version of OS are you using.



  26. Ravi says:

    Hi Neeraj,

    Removing the ‘#” character from the path worked. Many thanks for your post.


  27. nyrud1 says:

    This is the bizzarrest thing I’ve heard in ages.

    Removing the # from the full path???


    If you’d given me 100 other possible causes for the .chm not to work I would’ve picked the other 99 before this one.

    I wonder why this is…

    Never mind, it freakin works now


  28. Jatin says:

    It worked. Thanks a lot for the useful info.

  29. metrophage says:

    Nice. Thanks for the reg entry pointer. Fixed my problem.

  30. dpminusa says:

    I was going banannas with rhis error.  I have used the HTML Help Workshop for a couple of years and could not see what was wrong.  I poked around and rebuilt the project, etc., etc

    The problem was that I had a # in the path name.  I am using it for C# documentation so the path had the # along the way.  I changed the path name and voila!

    As a tip I would recommend you try the python sphinx documentor.  It will generate a Help Workshop project that you can tune from standard txt files.  

    It is best installed on a Linux OS.  I( never could find windows installation instructions.)  After generation you copy the directory to Windows and use the Workshop there. It saves a bunch of time.

    Good Luck.

  31. hussein says:

    i got this nice book in .chm extension and couldn't believe it could not open..  but the first trick did it for me!!! thanks man,, all d best!!!

  32. Sean says:

    That did it!  Thanks.  After several useless KBs, your registry fix was the key.

  33. HAHAHAHAHAHA!!! I have the best error!

    1) Take any CHM file. (ANY CHM file!)

    2) Copy it.

    3) Create a folder with the letters ".col" in it. (note: folder can't start with period, so do something like asdf.col)

    4) Put the chm file in that folder or any subfolder of it.

    5) It will give you the "cannot open file" error.

    Example paths that will cause this error:


       c:documents and settingsjohn.coltranedesktopTFSInstall.chm

  34. Anu says:

    WOW….Amazing… Thank you !!

    Option 1 worked for me.

  35. ranjith says:

    wow it's amazing,i just removed the # symbol from my folder,now its working,thx a lot guys….:)

  36. sandeep says:

    Your suggestion of using "unblock" really worked for me. Thanks a lot.

  37. Jack B says:

    It didn't work for me even after editing the registry and adding the keys.

    Just right click on the '.chm' file, select properties, and click on 'unblock' button!  Viola!! everything works!

  38. tkr says:

    'Unblock' did it for me.. Thanks dude..

  39. Piu says:

    Thanks a lot… I spend 2 hours trying to figure out the reason for this..

  40. Pradeep says:

    Thanks a lot…..it worked

  41. Jeff says:

    Ran into this irritating issue…thanks for the registry tip, worked a treat!

  42. Thomas says:

    Thanks very much. Solved the problem for me!

  43. Abbas says:

    Tried all option but it didn't open

  44. Nitin says:

    Thanks it worked for me after unblock

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