is there a way to convert a ptx file ?

Where did the PTX file come from? A common form is the uncompressed RAW image file captured with a Pentax camera which can be viewed using the Pentax software included with the camera.

If you're talking about this ...

RealLegal (E-Transcript of Court Proceedings) by RealLegal,LLCRealLegal E-Transcript Manager software is used to provide attorneys with high-performance, standardized, secure and signed electronic transcripts in the E-Transcript format. Once court reporters import their ASCII files into the software, they can activate full-text searching, condensed printing, saving options, and password protection for their encrypted work product. Attorneys receive the finished E-Transcript file, with the E-Transcript Viewer embedded or separate, via E-mail, diskette, or within a RealLegal Publisher Bundle.

(re: http://filext.com/file-extension/ptx )

... you'll have to use one of these apps to open it.

If they're not available for Mac OS, I guess you'll have to get one of them installed on a PC and either print or create a PDF.

Sounds like someone mixed up the suffix - try to use WORD to open it if you're sure it is a text file.

Try this -

http://www.reallegal.com/softwareDownloadETranscriptViewer.asp

That only runs in Classic mode (Mac OS 9 and possibly earlier).