How to add a downloadable form

upload it & put a link to it in a post or page

Where do I upload it too?

you can ftp it to: /wp-content/uploads and insert the link manually.

Or you can manage it from within the admin by using the media uploader in the edit post dialog – hit the add upload/insert media button (the star-shaped one) – remember to hit the button to create the link and the link will be inserted.

If you will have several downloads to manage, there are plugins available that could be useful such as Mike Jolley’s Download monitor.