Currently I'm working on Text to speech in android and I'm trying to build a document reader which would read the document for you but the problem is that I don't want to use default voice. Is there any way that I can use different voice which looks more realistic or human..?
There are Market applications that provide additional TTS voices. They need to be explicitly enabled in Settings though.
"SVOX Classic" is one example. It's an app that sets itself up as a TTS engine, and lets you download additional voices. Their focus, though, is on wide language support.
Android 2.2+ required - in 2.1 and below, there was no built-in support for third party TTS engines (not out of the box).