mod_deflate

mod_deflate is the apache-module that compresses output before sending it to the client.

<Location />
  SetOutputFilter DEFLATE
  BrowserMatch ^Mozilla/4 gzip-only-text/html
  BrowserMatch ^Mozilla/4\.0[678] no-gzip
  # BrowserMatch \bMSIE !no-gzip !gzip-only-text/html
  BrowserMatch \bMSI[E] !no-gzip !gzip-only-text/html
  SetEnvIfNoCase Request_URI \.(?:exe|t?gz|zip|bz2|sit|rar)$ no-gzip dont-vary
  SetEnvIfNoCase Request_URI  \.(?:gif|jpe?g|png|pdf)$ no-gzip dont-vary
  Header append Vary User-Agent env=!dont-vary
</Location>

http://httpd.apache.org/docs/2.0/mod/mod_deflate.html

if not excluding pdf from compression, this may lead to certain troubles when viewing pdf-files with a certain OS/browser/plugin-combination and the error “file does not beginn with %PDF-”