How do I delete a page from a PDF in Okular?

There is no way to remove a page in Okular. However, it can refresh modified PDF document automatically so there is no need to re-open the document manually after modifying it in external tool such as pdftk. To remove a given page use cat option and specify a range of pages you'd like to keep in the target PDF file. For example, if you want to remove 5th page in document.pdf execute the following command:

pdftk document.pdf cat 1-4 6-end output out.pdf && mv out.pdf document.pdf