Tanguy Ortolo

Traditionally, when you start a program to view a document, for instance xpdf resume.pdf, it stays in foreground, and only returns when it finishes, when you are done with the document. Of course you can still run it in background if you want.

Nowadays, several programs behave in a different way, for instance Evince. When you first start it, it stays in foreground as usual. However, if you launch it a second time while the first instance it still running, it will reuse this existing instance, asking it to open your document in a new window, and immediately return. Other examples are Firefox and LibreOffice.

Read more Viewers should stay in foreground