With WPPA you can also upload PDF documents. The document file is saved in the photo sources folder and will be available for display and download.
In situations where it is not possible to show the pdf file directly in the browser - e.g. thumbnails (too small) or the browser does not support the display of native pdfs ike in mobile phones - you should add a so-called 'poster' image file. This is a photo (.jpg or .png) with exactly the same name as the .jpg file, that you upload into the same album. The pdf and the photo wil 'merge' in the album admin; the photo will be used if the jpg can not be displayed. In case you did not upload a poster (yet) a default image will be shown: the documentstub image.
You can upload your own documentstub image in Basic settings -> Layout -> IV: Multimedia icon and stubfile specifications -> Item 7: Upload documentstub. Note: the file MUST be a .png file (e.g. a screenshot of (a part of) the first page). The upload sequence order (photo - pdf or pdf - photo) as well as the setting of 'avoid duplicates' is irrelevant
If you have ImageMagick installed on the server, and you activated it in wppa, AND your imagemagick version supports the conversion from pdf to jpg(!!), the poster image file will be created magically automatically when you upload the pdf document.
For more info on ImageMagick see this documentation page.