This is how I have added pdf datasheets to products (using VirtueMart 2.0.3a on Joomla 2.5.2).
1. Uploaded via ftp the pdf files into the product media directory - joomla_root/images/stories/virtuemart/product (you could use Joomla's media uploader for this, but ftp is quick for multiple files)
2. Open VirtueMart Shop/Media File list - and Synchronize media to VirtueMart
3. Still in the media list edit each of the files and make them downloadable, enter a suitable title to be displayed (Displayed image title) and I also enter the "Used thumb url" for a 50x50 pdf icon I have uploaded to use rather than the default larger icon.
4. Open a product, go to the Custom Fields tab and create a Custom Field Type - Photo (even though it is a pdf file), select the file from the dropdown list and save.
5. The file appears as my pdf logo on the product details page with its title underneath. The standard display is not good for datasheets, so I have edited the default_customfields.php template to suit my site and set it as an override. Further styling can be accomplished using css added (in my case) to the Joomla template css.