PDF missing charachters

[Dj YB]

On Tuesday May 3 2011 15:05:52 Tim wrote:
> On Mon, 2011-05-02 at 21:25 +0300, Dj YB wrote:
> > I have noticed that the Greek character Pi in some PDF documents is
> > replaced by the not equal sign.
> > how do I fix this?
> > should there be any fonts packages that I need to install?
> > which ones?
> 
> It could be that the PDF file is badly authored, but without an example
> nobody can tell.
> 
> Generally speaking, PDFs have their own fonts embedded into them, unless
> the PDF is using one of the few *standard* (for PDFs) fonts.  So a badly
> rendering PDF usually indicates a bad PDF file, or a problem with the
> reader.

Thank you for you reply.

the problem is not limited to a single document or even to a single source.

I am using Okular to read PDF documents.
assuming the documents are good (they have no problem in windows machines), 
what can I do to fix the problem with Okular not showing the right characters?

Thanks in advance,
YB.