Can a customer download a print ready (hi res) PDF from the site once they have placed an order?

If the product is an eDocBuilder product, you can use the option to "digitally deliver" the PDF (this option is on the product setup page under the Checkout section). But, when using this, the system will assume that you are not printing this job, but rather digitally delivering it. This will cause a problem if you are using an integrated shipping method and/or needing to charge for shipping based on weight. 
You can turn on the "Allow Downloading Proof PDF" feature which is found on the Template Info tab of the template in eDoc.  When your client selects the Approve/Checkout button during customization, they will be prompted to download a PDF proof before adding the item to their cart. 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.
If the product uses a standard forms-based eDoc template, then the user cannot download a proof pdf before placing the order.  However, you can enable the "Create Proof PDF" feature.  This is found in Sites > [site name] > Products > [product name] > General tab in the eDocBuilder section.  If you turn on this feature, 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 proof PDF will contain all the records (the person customizing the template only sees the first 5 records to insure they mapped the fields correctly). The proof PDF will still contain any Preview Watermark text that you have configured in the template’s Template Info tab in eDoc.
You can also allow a customer to download an asset file that is associated with a product, but that could happen at any time, and not just after the payment is made.