Annotations are not only limited to PDF files, they can be used for any format Okular supports.
Since KDE 4.2, Okular has the ́ ́document archiving ́ ́ feature. This is an Okular-specific for-
mat for carrying the document plus various metadata related to it (currently only annotations).
You can save a ́ ́document archive ́ ́ from the open document by choosing File → Export As
→ Document Archive. To open an Okular document archive, just open it with Okular as it
would be e.g. a PDF document.
Since Okular 0.15 you can also save annotations directly into PDF files. This feature is only avail-
able if Okular has been built with version 0.20 or later of Poppler rendering library. You can use
File → Save As... to save the copy of PDF file with annotations.