Symptoms:
In Internet Explorer, when you click on a link to a pdf file, you get the error message “Adobe Reader could not open ‘filename.fdf’ because it is either not a supported file type or because the file has been corrupted.”
This error is due to an issue with Adobe Acrobat viewing PDF files in a web browser.
Try the following:
1. Open Internet Explorer, go to Tools -> Internet Options and on the General tab to erase all temporary files, cookies and history data.
2. Under Internet Explorer -> Tools -> Internet Options, on the General tab click on Settings and make sure you select the option “Automatically” for checking for newer versions of the stored pages. Then attempt to open the PDC file again.
3. If solutions 1 and 2 do not work then uninstall Adobe Acrobat, then reinstall it again.
It seems that Acrobat 6 is more prone to this error so upgrading to Acrobat 7 may solve the problem.
A day in an IT technical assistant's life 
August 8, 2007 at 1:04 pm
I get the above mentioned error only sometimes in IE.
Why is it so?
It should have shown me the error in other places too..
Kindly respond.
August 9, 2007 at 12:56 am
Actually, in my case too, the trouble hapenned with only specific .pdf files, and not with others.
I’ve been unable to tell the reason why, but the above solution fixed my problem.
November 14, 2007 at 11:46 am
I tried the same option. It is working fine.
November 14, 2007 at 2:15 pm
fdf is a format / content file which can be used to alter the way a pdf look, it can for example contain values for fields that are in the pdf.
I use them to generate customer information in fdf format, then load the fdf which in turn loads the pdf with most the fields filled out allready. Therefore if a pdf is just text, chances are no fdf, if it can be filled in, or has complex layout then it might be supported by a fdf file.
http://support.adobe.com/devsup/devsup.nsf/docs/51044.htm