1. A method of converting an input document having a fixed-layout into a portable format, the method comprising, for each page of the input document: extracting a font used on the page; adjusting metrics of the converted font; converting the extracted font into a format compatible with a web browser program; extracting text from the page; rendering content of the page, other than the font and the text, as one or more content images; storing the converted font, the extracted text and the one or more content images; combining the converted font, the extracted text and the one or more content images as a corresponding page of an output document formatted according to a markup language compatible with the web browser program; comparing an image of the page of the input document to an image of the corresponding page of the output document to generate an error score; and if the error score exceeds a threshold, replacing the corresponding page of the output document with an image of the page of the input document.