There are lots of PDF submission sites available in google. You have to search better sites from there. Before submit PDF, just take care of your content. i mean if you submit PDF in any related sites then some sites just fetch all your content and which is not good for SEO purpose. So try to find sites which not fetch all your PDF content.
I have best sites which not fetch content. Try these two: issuu.com and edocr.com
PDF submission is an off page technique used to build backlinks for your website and also generate traffic for your domain. The process helps to build your website popularity in search engines.
you can get a wide list in google: However my personal experience gives the following as the good ones...They are more than sufficient to start with to drive quality and potential users.
As @djone said, there is lots of PDf submission sites, I am listing down some popular sites slideshare.net, scribd.issuu, docstoc, 2shared, authorstream, filefactory.
Abstracts must be submitted in English within the following structure:
Purpose – Materials & Methods – Results – Conclusion.
Abstract length is limited to a maximum of 200 words (no graphics, Pics, Tabs).