When you visit a Web site that contains a link to a file that will be
opened outside Internet Explorer, and the Web site also includes the VARY header in the response to the client
GET request, you may receive the following error message when you click the link
and then click Open
the file path of a temp file.
The VARY header instructs the client not to use a cached
file. Therefore, Internet Explorer does not write the .chm file to the hard disk. HTML Help
requires a file. The VARY header response from the server is similar to
"vary: accept-encoding, accept-language, cookie,
Internet Explorer does not fully implement the
VARY header per Requests for Comments (RFC) 2616. The Internet Explorer implementation of VARY is that it does not
cache any data except for Vary-Useragent.
This problem is known to occur with HTML Help files (.chm files) and vCard
files (.vcf files). Other programs that are not listed in the "Applies To" section of this article may also exhibit this problem.
Programs that open files directly in Internet Explorer, such as Adobe Acrobat, do not exhibit this