I'm in the process of starting a new website that will involve me contacting several third parties for a small involvement in this project. This will not be a sponsorship request, but more of an agreement on recieving and displaying content from such third parties.

Seeing as though i'm on a shoestring budget (therefor a formal letter to 100+ company's will be out of my price range), i'd like to ask the following question.

What would you consider the more professional approach to such a request?
Plain text email, HTML email, email with PDF attachment, email with link to PDF file, or any others?

I'm pretty sure most people will pick the PDF in some way, so are there any easy ways to go about creating a PDF file with minimal outlay?

I vote for plain text email.

Yes. I think you should do either a plain text or HTML email, and if you wanted to take it a step further, you could put a PDF attachment at the bottom of the email. And as for making the PDF file, there are a lot of websites you can find that will help you to make them. Just search the web for "creating PDFs" etc..

I would prepare a plain text letter. Call to the company ask for contact of decision maker and send the letter to the right destination.