Converting of PDF document to MP3 audio is possible, in case you need to convert written text a voice message. In order for this to work, you will need Adobe Acrobat, as well as some audio recording software of your choice.
In Acrobat, open your text document and use View ► Read out loud function. You can read whole document or only part of it and the program will read your text. Probably works only for English language.
Before you start the reading, you will need to setup sound recording program that will record the voice from Acrobat and save your text in form of audio file, in this case .mp3.
Alternatively, you can use some of the text to speech conversion solutions, but that may require copying the text out of your PDF document to a plain text format first.