Hello 18ME
You need a Product Type with File (or Language Dependent File) set as one of the attributes. It's possible that the "Standard" Product Type may already have this (assuming you haven't changed it). If not then you'll have to create one. Go to Product Type > (create new product type) > Attributes. Create new attribute with type set to File (or language dependent file).
Create PDF file on your computer hard drive. Name file using keywords to help with Search Engine Optimisation.
Go to product you wish to attach file. General tab > Attributes. Upload file.
It's not so obvious in your shop front that a download is available. You may have to refer to it in your product description to bring it to your customer's attention. See this example:
http://www.rikkismusic.com/Casio-Privia-PX-130-Digital-Piano-Black
