WordPress PDF viewer plugin helps your site visitors to preview a PDF document before they download it. The default viewer with WordPress Download Manager Pro usage google doc viewer which only can preview public PDF documents. But you may use this viewer to show protect/members-only documents too. When you have multiple PDF attached with a package, use template tag [pdf_gallery] to show all PDF previews in a gallery view.
Download the plugin from the premium download area
Install and Activate
To show a single pdf preview use template tag[doc_preview]in your page template.
Password Protected PDF view
When you have multiple PDF attached with a package, use template tag[pdf_gallery].