How can I annotate PDFs so notes are compatible with Adobe Reader for Win/OSX?

I am looking for a tool which allows me to annotate PDFs in Ubuntu and share the PDFs with annotations with people who use OSX or Windows. I cannot assume that the others can or will install special software to read the annotations.

I am therefore looking for software which allows me to make annotations in PDfs which are compatible with Adobe Reader.

What good alternatives for Ubuntu exist for this?

Look for the package "xournal" in the software center or user apt-get to install it. With it you can annotate PDFs and export them as PDF again.

Besides Xournal (see m4lvin's reply), you could use PDF XChange Viewer. The free version allows for annoting PDFs, too; it runs without problems using wine.

Newer versions of Okular can save annotations directly to the PDF file.

By default, the annotations are saved in the local directory for each user. However, if you choose "File" -> "Save as..." you will be able to save your annotations in a new PDF file, so that they are visible by any other PDF viewer. (See http://okular.kde.org/faq.php#annotateandsendtofriend )