I believe you have still not read through the documentation links which gives steps on how convert different file formats to PDF :
I have gone through the documentation & i had enabled PDFExportConverter... In spite of this its not working
When you check the status/result of the IBR conversion job, what does it say? I'll strongly assume there's an error there.
A common gotcha is that people don't install the i686 linux libraries for conversion server's OIT. But, this will show errors if you're looking for them.
did you complete the file format configuration on content server (double check if the conversion is enabled on CS and IBR)?