Can I convert my old 2005 .tax file to .pdf?

If you did not save the 2005 return as a PDF and do not have the 2005 program to open it then you may consider using the service on this website - http://www.taxprinter.com/

This is not a recommendation so use at your own risk.

It can't be done via Turbotax.  There are a couple of web sites that claim to keep old tax software running on old computers to be able to do this for a fee.  We can't vouch for the security or safety of any particular company.