Some people get access to the site no problem, others are being asked to download a file instead, anyone know what causes this?
I think it's going to be related to mime type. The headers from the server should specify the mime type, in some cases browsers behave differently for different mime types and they have different fall backs for different files.
I had an issue with .exe files (used within the frontpage admin website) for example; in IE they were by default just treated as text/html, firefox however tries to download because it doesn't actually know what they are.
AddType text/html .exe
in apache config solved that little issue. it made Firefox realise the .exe files were actually text/html.
Is this your own server or a shared one? You could check the settings in somewhere like /etc/httpd/conf.d/php.conf and mime type settings in httpd.conf
see what the server has setup for .php files (or what ever is giving you the problem).
The http headers coming out of your main page show you're using gzip page compression and "chunk" transfer encoding. It's likely some browser clients can't deal with it. The server never knows what happens after it sends a compressed page, you just get an OK 200 code in the http log, whether the client was able to unzip it or not.