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The ? Normally means that the character in question does not exist in the font being used by the reader. It could be that when you look at it using the calibre viewer then the character does exist in the font file on your PC.
The commonest cause of this is where so-called smart-quotes are used in the source file as these smart-quote characters are outside the normal ASCII character set and thus may well not exist on the target device. Getting the correct character encoding specified can fix this issue, as can the calibre option to handle smart-quotes at conversion time (by converting them to normal quotes).
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