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vsolov - In general ebook formats (EPUB, MOBI, AZW etc) don't/cant have page numbers because of varying screen sizes, font sizes etc - they're known 'continuous flow' documents, as opposed to 'paged' documents such as PDF, DOC, ODT etc.
The reader has a bookmark feature, maybe it could do you want, see ==>>
http://manual.calibre-ebook.com/viewer.html
BR