conversion rates on php pages

Ok, I have only been at this SEO lark a couple of months, although I am beginning to understand the overall concepts of how to optimise a site and make the most of adwords, there are little niggly things that keep puzzling me. So I apologise if this is a silly question but hopefully someone will be able to advise!

I need to activate conversion tracking on my sites, and for us a conversion is when someone downloads some information. To do this you fill in a form, the form is processed using a PHP file, and then a PDF opens. I am pretty sure I can't put conversion tracking on a PDF (can I?!) so will it be possible for me to put it on the PHP page that processes all the info on the form - will it require anything special since it's php? this is the only logical place it can go - there's no point putting it on the form page as so many people will open it and close it once they realise you have to give info to download stuff!
Any advice much appreciated.

Also, how long would you recommend giving conversion rates before using them to make the PPC campaign more cost effective?

cheers x

I would add one page to your system.

After the person fills in the form and presses the submit button, you can serve a page that has a download button and the tracking tag.