If you create Joomla! User Groups corresponding to your shopper Groups then it is straight forward to have Downloads that can only be downloaded by the relevant group member.
Suppose these User Groups are called say shopper1-ug, shopper2-ug and so on set Registered as the parent of these user groups. Further if you create an Access level called say shopper1-view whose only member is shopper1-ug; an access level called say shopper2-view whose only member is shopper2-ug; and so on then only members of the relevant user group will be able to see thier price list.
In jD you could have an individual Category for each shopper group , called say shopper1-cat and so on. For Shopperi-cat set the view access to shopper1-view and the Download permision for shopper1-ug as allowed. Any Download you create in shopper1-cat would inherit those setting and only be seen and downloadable by members of shopper1-ug.
An important aspect is to also change the default jDownloads Download permission for the Public user group from Allowed to Inherited (NEVER use Deny as it has many unintended consequences due to Joomla! inheritance rules). You can get to this by going to the jD Control Panel and using Options button near top left. This wil open upthe 'component' jD permissions for the Public ug and you will see Downloads permission set in green as Allowed - change to Inherited and then save & close.If you have other Downloads that are in other Categories then change the permisions for the Public ug to Allowed in each such category. Always do a Save.
ColinPS the Documentation has several articles on this topichttp://www.jdownloads.net/documentations/category/jdownloads-version-3-2
will may now see a warning notice in jD Control panel about not having any global 'component' permissions set but you can ignore that message