Problem with showtext method

Hello,

I want to do a pdf with lines containing opened accents and cedillas using the PdfContentByte method showText. I  call this method with a String with words with these kinds of accents and the result isn't the expected one: in the output pdf file these accents are interrogations.

Anyone can help me with this problem? Can I use another method for this task or I am not using showText correctly ?

Thank you very much and sorry for my English
Laura