Question: Our client has several specification sheets in pdf format for many of their products. What is the best or easiest way for them to add a downloadable PDF to each product page. (Some PDFs fit many products).

UPDATE: I found a very useful extension for this <http://amasty.com/product-attachments.html>

Answer: 
I think just uploading them and using the WYSIWYG Editor to add a link is not enough?

How much PDFs are we talking about?

You could add a new multiselect to the products, add a source model und upload the pdfs (have a nice grid, etc.) and select them in the product.

If there are too much, you can implement your own chooser with a js-grid (incl. search, etc.) and JS magic. I did that once for a product-chooser (`Mage_Adminhtml_Block_Catalog_Product_Widget_Chooser`)
Another possibility is to use an extension which does exactly what you want to do.
There exists a free extension for this purpose:

<http://www.magentocommerce.com/magento-connect/pdf-for-every-product-8631.html>

Another one which is free for up to 1000 products is this one:

<http://www.magentocommerce.com/magento-connect/pdf-for-every-product-8631.html>