Amazon Kindle Fire only supports azw/prc/mobi/tpz. Epub and pdf can not be natively accepted by Kindle Fire. So you need third-party program convert it.

You can google for example PDF MOBI CONVERSION to find programs (most are not free) to do that conversion. But note that some pdf's and epub's are protected by digital right management (drm). Our forum policies don't allow to discuss removing that.