Can a customer download a proof pdf?

Yes. You can do this if you select the option "Allow Downloading Proof PDF" which is found on the Template Info tab when setting up the template in eDoc.  For the Interactive Designer, this option is available when your customer selects the Approve/Checkout button during customization, they will be prompted to download a PDF proof before adding the item to their cart. In a Form-Based template, this option will become available after completing the required fields and Updating Preview.
Note: If you plan to use this option, we highly recommend that you also enter Preview Watermark Text in the Template Info tab as the proof the user downloads will be the same resolution as your production file. The Preview Watermark will put a translucent text block on the proof PDF that the customer downloads.
For both Interactive Designer and standard forms-based eDoc template, you can also download a proof pdf from the Saved for Later area if "Create proof pdf" and "display eDoc proof in cart" are both turned on for the product.  This setting is found in Sites > [site name] > Products > [product name] > General Information tab in the eDocBuilder section.
Remember that Save for Later is an optional feature for your site.  It is turned on in Sites > [site name] > Settings > General info tab in the Site Basics section.
If you enable the "Create Proof PDF" feature (found in Sites > [site name] > Products > [product name] > General tab in the eDocBuilder section), the user (or an approval user) will be able to download a proof PDF of the customized eDocBuilder product after checkout. When the approver logs in to approve an order (or when a user has finished checking out), underneath the preview image, they will see an option to download a proof PDF of the order.
If this template uses a variable data upload (for instance, a mail merge), the customization screen and the proof PDF will contain the first five records as a sample. Once the session is finished and the order placed, the production PDF will contain all the records from the variable-data file.
The proof PDF will still contain any Preview Watermark text that you have configured in the template’s Template Info tab in eDoc.