Considering that your software is already heavily based on the open source stack (it is clearly based on Qt4 and KDE4 libs) it seems it would behoove you to open source it. KDE4's reader (okular) is better than the Mendeley one, there is no reason why Okular and Mendeley couldn't share these improvements. Additionally, it would give back to the community that is making your business possible while not hurting you in anyway since you make your money on the document storage and syncing service.