I have read an article on the Web that it is possible to create PDF thumbnails using ImageMagick, will it be possible in Omeka?
this is the article - Create PDF thumbnails using ImageMagick on Linux
In relation to this, the iPaper plugin works only if you have a valid domain and connected to the Internet but doesn't work in a development server using localhost.
Is there a way to connect Omeka to external applications using plugins, like in this case Xpdf, Evince, KPDF or other PDF viewer?
Yes, it would be possible to make a plugin that created PDF thumbnails using ImageMagick. Currently we have no plans to add that as a feature in Omeka, but we also didn't know that was possible, so thanks for the heads up.
There are at least 2 ways that I can think of to make a plugin that would interact with PDFs uploaded through Omeka. You could either hook in to the file upload process to create a thumbnail or image (the 'after_file_upload' hook), or you could write a template helper that would use a 3rd-party web app to display the PDF in your Omeka theme.
Feel free to let us know if you decide to embark on one or another of these plugins. We recommend that you post to the dev list for help with development, as these forums are more geared towards support for existing software.