[[Class_software#Xournal_-_GTK.2B_inking_application|Xournal]] is a GTK+-based inking application. It runs on Linux and potentially on other platforms. Annotation of PDFs is supported by loading a [[w:Poppler (software)|Popplar]]-rendered PDF as a background image, but it requires that annotations be saved to a separate .xoj file or exported to a new PDF file, and does not support annotating PDF files directly.