Is it possible to convert .pdf documents to .mp3 format?
Converting Adobe Portable Documents directly to MPEG Audio Files (.pdf to .mp3 conversion), per se, is not possible, as PDF documents do not contain multimedia data.
You can, however, convert written texts from inside PDF documents to speech audio files, in MP3 format, for example. To be able to do that, you will need Adobe Acrobat, as well as one of many available audio recording tools. In Adobe Acrobat open your PDF document and select 'View -> Read Out Loud' function.
As an alternative, try one of numerous text-to-speech conversion utilities, which may, however, require copying the text to a plain text file first.
A text-to-speech software
A text-to-speech software for Mac