How to install 32-bit version of libstdc++.so.6 on a 64 bit machine

You could've used apt-get install lib32stdc++6.

What makes you think you need a 32-bit version of libstdc++.so.6?

64-bit Ubuntu runs 64-bit Apache which runs 64-bit PHP which it seems can't load 32-bit libpdf_php.so, which it tells you in the error message.

It's unlikely that installing ia32-libs will solve the problem - I would say you need 64-bit version of libpdf_php.so

It looks like pdflib's website has a separate download for x86-64 architecture - I downloaded the archive and the .so file inside it is definitely 64-bit.