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Are there any rulrs-of-thumb egarding size of Epub vs pdf with the same content?

I'm well aware that this depends on the content, encoding, compression and so on, but for e. g a 3mb pdf report on around 100 pages with a few images, what could you expect with regards to file size for the epub? More or less and by what factor?

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Generally the epub will be about 10 times smaller. With a PDF, not only is the text stored, but so is the location and orientation of each character. In the case of an OCR'd document, each page is stored partly as an image.

On the other hand, EPUB is just HTML. There are no rules regarding where the text should go--the EPUB client has full power over reflowing the text and/or rescaling. Due to this, a 3mb PDF will correspond to a 300kb EPUB approx. If the 3mb PDF is OCR'd scans, then the EPUB will be even smaller.

Images in PDFs are a teens bit smaller, though.

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