Since very few MR members read Tamil, you'll have to be more specific regarding the problems that you're facing.
If you're trying to convert a .pdf file to an .azw3 file with Calibre you shouldn't expect perfect results, because even utf-8 encoded Latin alphabet based .pdf files rarely convert well.
This particular .pdf also won't convert well at all, because it's not encoded in utf-8 but TSCII. You may want to search for the original TSCII etext, convert it with an TSCII to Unicode converter and then convert the Unicode text file to an AZW3 file.
If that's all too complicated for you, try to resize the original file for 6" screens with
k2pdfopt.