With version version 5, Master PDF Editor has removed some features from its free to use version, like editing or adding text, inserting images, and more - when using such tools, the application adds a big watermark to the PDF document unless users buy the full version (around $83).
Master PDF Editor 4 though, which is free for non-commercial use with no restrictions (at least on Linux, I'm not sure about macOS and Windows), can still be downloaded and works well, even though it's not linked on the application official website.
