This article shows how to use Text to Speech APIs:

In RDPDFViewController you can find OnSelEnd method that gives you the selected area, in this case you can reproduce the selected text with TTS APIs:

- (void)OnSelEnd:(float)x1 :(float)y1 :(float)x2 :(float)y2
{
    if (m_bSel) {        

        // Get the selected Text
        NSString *s = [m_view vSelGetText];
        if(s)
        {
            // Initialize the TTS
            AVSpeechSynthesizer *synthesizer = [[AVSpeechSynthesizer alloc]init];
            AVSpeechUtterance *utterance = [AVSpeechUtterance speechUtteranceWithString:s];            

            // Set the speach speed
            [utterance setRate:0.3f];

            // Set the language
            [utterance setVoice:[AVSpeechSynthesisVoice voiceWithLanguage:@"en-US"]];            

            // Start playing
            [synthesizer speakUtterance:utterance];
        }        
    }
}