[Zope] Apache 2 : stuck fixing content-length HTTP header

Hi, This is not - strictly speaking - a Zope problem, but perhaps it's a fixed issue for some of you. When using the standard RewriteRule scheme with a VirtualHost monster, Apache 2 removes the "content-length" HTTP header from the response provided by Zope. This does not matter for most web pages, but this disturbs viewing a PDF file with IE (for a reason I don't know). Mozilla (and perhaps other browsers) don't care about this and show the PDF file through the plugin. But most people use IE and I didn't find anything in the Apache 2 doc to fix this. Any hint is welcome, including RTFMs :o) Many thanks in advance. --Gilles