When comparing Xournal (Linux) vs PDF Expert, the Slant community recommends Xournal (Linux) for most people. In the question“What is the best software to annotate a PDF?” Xournal (Linux) is ranked 7th while PDF Expert is ranked 9th.
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Pros
Pro Open-source and free software (GPL license)
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 Fast and smooth
It is a software focused on consumption and not in editing PDF (yes, you can accentuate highlight, draw shapes and sign). They managed to build a nice looking efficient product in which they propose to do.
Cons
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.
Con Expensive for what it offers
It is an item that improves aspects of PDF reading (which is free: Preview) but lacks professional features that other similar software has.
Con Missing settings options
Lacks basic settings like anti aliasing level.