Is it possible to make a whole different Document Viewer, That would display documents and images. I was thinking something based on Adobe Acrobat reader. A plug-in that would display a file and have a function of zoom in and out, drag-gable function and have a menu for multiple files within 1 item. To give an example here is the ContentDM site that has newspapers archive: http://contentdm.clark.edu:81/cdm4-ccsn/document.php?CISOROOT=/p280202ccsn&CISOPTR=158&REC=1
That's the only one good thing about ContentDM.
That page you linked to looks like it just directly embedded the PDF within the page, and your browser is handling the display.
Since you mentioned it specifically, I'm guessing you were seeing the Adobe Reader plugin on that page. Other users might see PDF plugins provided by their system (Chrome includes one), or nothing, if they or their system doesn't have a PDF viewer (or has one that doesn't provide a browser plugin).
An Omeka plugin that just does a simple embed for PDFs and leaves the display up to the browser would be easy to write. All it would need to do is output a simple HTML
object tag for displaying the PDF.