Maybe you can use a tooltip (on Windows and other OSes, that's a yellow square box that appears when you hold your mouse over a specific element)?
In HTML, most (all??) elements have a title attribute that you can use:
<td title="This is a long description">Short Desc.</td>
Linking to a PDF is exactly the same as linking to other content:
If you want to open the pdf in a new window, add a target="_blank" attribute:
<a href="YourFile.pdf" target="_blank">Download</a>