Page numbering is a convention from the world of paper books. Its position in the eBook world is yet to be settled. While page numbering in paper edition of books is straight forward, this is a more complex issue in the eBook world since it is uncertain if and how such a feature would be best implemented.
The use of page numbering would be the same as in paper books - text navigation and quantification.
Different approaches might be used:
 Absolute page numbers
Absolute page numbers would be used to refer to a particular portion of a book in a way that would be consistent on different screen and font sizes and independent of the format of the eBook. In order for this to work, the page numbers would have to be embedded in the mark-up of the text or a page length consisting of a specific amount of text/bytes have to be agreed.
Absolute page numbers might be helpful to readers who need to refer to a specific position in an eBook, either for references in scholar works or to look up the page on a different device.
Absolute page numbers might also be helpful to reference the length of an eBook or a part of the eBook.
The page numbers to be used in eBooks could be aligned with paper book versions of the same book for easy referencing. In cases where multiple editions of the same book are available and the page numbering varies in the different versions, the publisher would have to decide which paper book edition to use as the source of the page numbers. Hardback editions and paperback editions of a paper books often have different layout and page numbering.
 Rendered page numbers
Rendered page numbers refer to the page numbers in an eBook as it is rendered in particular format, device, and font and is helpful for the reader to navigate in this context. Rendering page numbers may or may not be independent of font size changes.
Rendering page numbers that change with font size is typically using a screen display as a page. The total pages is the number of screen display changes what will be needed. This follows the idea of pages in a paper book but can, in no way, be used as a reliable page reference. These page numbers are generated on the fly and may increase the start up time if they attempt to provide a total count. Some older devices just count as they go and will just make a guess if you jump ahead. They will also change if you switch from portrait to landscape mode. Progress may be indicated using a percentage.
Page numbers that are rendered the same irregardless of font sizes (and likely margin changes or other formatting changes) will typically be based on some measure such as a set number of characters. This number may be larger or smaller than the actual number of characters that appear on a screen. They can be used as a reference for a group of people whose eReaders use the same convention. In this case screen size of the display does not change the count. There can be multiple pages on the same screen or several screens of data can be on the same page.
The idea of page numbers in eBooks has been criticized as being obsolete and non-logical. Paragraph and word numbering has been suggested as an alternative to absolute page numbers while percentage has been suggested as an alternative to rendered page numbers.
The EPUB format specification version 2 from IDPF does not contain page numbering. Adobe has developed an add-on feature, Page-map, for publishers to use in defining page numbers in their eBooks, described in Adobe's EPUB Best Practices Guide (epub). In the absence of a page-map ADE uses 1024 characters to represent a page.
The Sony Reader in its LRS format and the iRex iLiad shows rendered page numbers. The ADE ePub format used on Sony and many other readers generates a default page number scheme based loosely on 1024 characters of the book per page number. This usually means several screens worth of data will represent a single page and that page numbers may change in the middle of a screen.
Mobi readers use a screen as a page number however no MobiPocket Reader including the one on the iLiad are accurate when it comes to page numbers. They are always sophisticated estimates that fool some users, however if you jump ahead many pages it will be off. The eBookwise-1150 accurately renders page numbers as screen pages.
The Kindle uses an absolute position system named locations which are actually every 150 bytes in the file. Easy to generate accurately but are much smaller than a standard page so you get a range of numbers on every page. The exact formula is floor(bytes/150)+1, and note that this is bytes, not characters, and includes all the markup and tags in the mobi-html, not just the displayed text. The Kindle 3 and later models have an absolute page number capability using a new side file (APNX) that contains a map to the numbers. This data can be encoded in the DAISY page number section of the NCX file.
The ePub 3 standard does provide for page numbers using a variation of the Navigation section. It effectively replaces the DAISY implementation as there is no DAISY connection in the version 3 standard.
- http://www.idpf.org/accessibility/guidelines/content/nav/pagelist.php - page list
- http://www.idpf.org/accessibility/guidelines/content/xhtml/pagenum.php - page numbers
PDF files are typically designed one for one with page numbers on the paper book but even there there may be different starting points so that the number numbers may be offset from the ones in the book. In addition a PDF that has be generated in a different size from the paper book can have the same problem as ePub except that often the page number from the book are still seen on the document due to transferring the header data.
 Related Links
- Page numbering, paragraph marks, whatever—just how should scholarly publishers point readers? by David Rothman at TeleRead
- Fixed layout ePub is an implementation where the exact page layout is replicated, including, of course, the page numbers.
- Adobe Page Map vs. NCX paglist - from threepress.org