Disclaimer: I work at Atalasoft
We sell a SharePoint solution (Vizit) that has PDF rendition and annotation capabilities: http://www.atalasoft.com/products/vizit
There is no support for doing this in SharePoint alone unless you implement a solution. You'd need to turn PDF into something web-browser compatible, and then implement the editing features you need.
If you have Acrobat installed on clients, you may be able to do what you need by using their ActiveX controls. The trick is getting the edits back into SharePoint and playing nice with version control, etc (Vizit does all of this for you).
If you just need reading, you can serve the PDF with an HTTP header called "Content-disposition" set to "inline" -- they will not be able to edit.