The ePUB file type is an open source format developed by the International Digital Publishing Forum (IDPF) . Based on XHTML, XML and CSS it works well with a number of software applications which has helped the popularity of the format grow. Despite this, there are a raft of rules that must be adhered to when creating an ePUB file which has attracted criticism of the format. The format allows DRM protection to be added to a file.
The ePUB format allows reflowable content. This means that the content fits the size and shape of the screen it is being displayed upon, in the same way that text on an HTML page changes depending on the size of the display. An ePUB file tends to consist of three elements: archive, metadata and content.
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The .pdb file extension is a database format used for the Palm OS platform. The PalmDOC eBook file type is a version of the PDB format. The proliferation of eBooks has left this format behind due to the lack of support for DRM protection meaning only royalty free text is typically available.
The format supports reflowable content however the format lacks some of the features found in more modern eBook formats, for example only providing a very basic contents table. The format features RLE compression which was crucial because the format was designed for the low storage capacity Palm devices of the 1990s.