To use percentage or page numbers? This is a problem.
When reading EPUB books on electronic devices, we notice that a lot of apps display the current page number as paper books do in the rendering of reading content. Unlike PDF files which are layout based, EPUB files are streaming. In other words, the resolution of the device has a direct impact on the EPUB books pages. On different devices, the total page number is not the same.
In the case of insufficient resolution, the layout formatted PDF files can be zoomed in or out and read by roaming fingers on mobile devices. This isn’t good reading experience although the page total is always fixed, especially when the resolution is not big enough. The advantage of streaming EPUB files is that, no matter in what resolution, a font size suitable for reading is always available. The only thing you need to do is just a simple taping, a benefit as the result of resolution considerations in the EPUB standard for mobile devices.
Usually an issue arises when displaying page numbers in EPUB rendering: the need of calculating page numbers dynamically according to the device resolution. And that results in additional memory use and some doubts due to contradiction to habits in the case of displaying percentages. Take iBook as an example, during the loading of a book, you will note colour in the progress bar below turning gradually from grey to black. It’s an indicator that the page calculating process is underway. It will eventually produce a scroll bar which users can move quickly, and page number will appear on the top.
Imagine such a scenario: my friend opens the same book on an Android device, and his page number, because of resolution difference, is not the same as mine. When I say, “turn to page 57, there’s an interesting story there.” My friend won’t find the story.
If I say in another way, for example, “turn to 57% position”, or even more precisely by omitting the percentage and saying “turn to 57.23″, then this can be quickly positioned by inputting the digits, and the problem just described will be no more.
About the issue of additional memory use, the usual page calculation takes resolution into consideration. Calculation mount will be much smaller with percentage, saving very precious battery power.
Another problem, for Apple (the same for Amazon, Google…), it’s best that EPUB books are displayed on their own devices. It’s the rule they dream for the world. In the real world, however, we have a lot of good friends who have different preferences for manufacturers. We do not want such preferences affect their pleasure of reading.