The question is not necessary for newer versions of Okular. From FAQs section of the Okular website
Why the newly added annotations are not in my PDF document?
How can I annotate a document and send it to a friend/colleague/etc?
They explain that in newer versions of Okular, annotations can be saved directly into the PDF so not in separate metadata files. Some of these annotations can also be read by other PDF readers. The FAQs also explain that Okular PDFs can be saved as .okular files, for other Okular users to edit. (Although I do not see the need if annotations can just be saved into the PDF.)
Importantly previous annotations made with metadata method can be saved into the PDF. Open the PDF with metadata annotations, then 'save as' and the document will have the annotations saved into it. Therefore the annotations are viewable to anyone who can access the PDF via Dropbox that you have saved it in.