How to add PATH for readPDF {tm}

I am struggling with readPDF command from package 'tm'. When I run the following command:

docsPDF <- Corpus(DirSource(cnames), readerControl = list(reader=readPDF))

      

        
        
        
      

    

The above command gives the following error:

Error in system2("pdftotext", c(control$text, shQuote(x), "-"), stdout = TRUE): 
'"pdftotext"' not found

      

        
        
        
      

    

This is because the XPDF files "pdfinfo" and "pdftotext" are not present in the R system.

file.exists(Sys.which(c("pdfinfo", "pdftotext")))
FALSE FALSE

      

        
        
        
      

    

I'm just wondering how to add the paths of these two files to the R system (under Win8)? Thank!