Writing notes on pdf’s

Now I can do it.  I can read a pdf document on the screen, highlight or underline parts of it, write notes in the margins, and save it.  I can change or erase my marks without changing the document.  I’ve been looking for a way to do this for a while, and I can do it with Xournal. (It is said that some other programs can also provide this capability, but I haven’t tried them.)