It hasn't been updated since 2017, but there is an active fork Xournal++
You can annotate PDF as if you were using a paper, without special textboxes as in other PDF editors. In addition, Xournal saves your annotations in a separate XOJ file, which is really small compared with the original PDF. Having a separated file for annotations, I can share the PDF without sharing them, and I don’t need to duplicate the PDF, saving space. Downsides are an outdated interface, and a rare way of using tools.
Ocular app is very good but I got surprised about Xournal , even better ... Can export PDF notes to PDF .http://www.linuxinsider.com/story/74882.html
Easy way to make small additions to PDFs
Great app for PDF annotation. It will make the text remain selectable and copyable.
Our users have written 4 comments and reviews about xournal, and it has gotten 63 likes