Here's how you generate a tear sheet from a proposal:
At this time you are not able to download tear sheets directly from the product library. You can only download a tear sheet for a product after it has been added to a project (i.e. put on a proposal, invoice, or PO, or added as a draft item directly to the project tracker). Once the item is on one of those documents or in the project tracker as a draft item, it will appear in the project tracker for that project, and from there you can select the product and download the tear sheet for that product.