At the moment CSS forum is not offering any kind of uploading facility i.e files cannot be attached to your posts.But you can upload your documents and files on external websites and share them with other members on CSS forum.
Firstly,you need to create an account on File Hosting sites where you will be directed to upload your documents through a step-by-step guide.After uploading,you will get a URL(download link) that you can share here on cssforum through your posts/threads.
Note:No need to convert any file.These websites support almost all kind of files such as .Pdf,.doc,.txt,.zip...etc.
Recommended Free File Hosting Websites:
www.Mediafire.com
www.4shared.com
www.box.com