Xournal is an application for notetaking, sketching, keeping a journal using a stylus.
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Pro Supports Wacom tablets and similar input devices
Detects the pressure applied to the pen and adjusts the line thickness, creating more natural-looking drawings.
Pro Open-source and free software (GPL license)
Con Importing a PDF can be a lossy process
Upon importing some PDF files, each page becomes a raster picture. Features of the original PDF, such as vector graphics or selectible text, are completely lost.
For other PDFs, this problem does not happen.