Some characters extracted from pdf hasn't corresponding unicode in its font

Acrobat does not use Unicode to show the characters in a PDF, so this will always work. Unicode is a layer added during text extraction, and sometimes this will fail, either because there is no mapping defined in the PDF, or because the characters have no Unicode range.

No, Acrobat uses the embedded fonts. You can clearly see this on screen by using unusual fonts.

Unicode is not used, and not needed, to show embedded fonts on screen. Either Encoding or CMap are used to directly find characters in the font. The PDF specification has full details.

.

You cannot. Probably. Many PDF files do not permit accurate text extraction.

A simple test is to see if Acrobat can extract text accurately. Acrobat has 20 years of development in this area, so if it cannot get the text, it is probably the case that you cannot either.

1. Extract text using Acrobat

2. Look at it or analyse it.

Sorry, I know that seems an unhelpful answer but I don't understand what else you can mean.

No, Acrobat is not free, but you can do the same test with the free Adobe Reader, using Copy/Paste to extract text.