The Digitale Sammlung Landesbibliothek Öberösterreich is the digital collection of the Landesbibliothek Öberösterreich in Linz. In the holdings of this regional state library you can find a substantial number of digitized works related to the First World War. You can search for works using either the simple search or the advanced search mode. The website can be viewed in German and English.
At a webpage on the First World War of the German text portal Wikisource you can find many titles digitized at Linz (below “Weitere Quellen”, abbreviation OÖLB).