how do i change .spx to pdf?

Do you know which kind it is?

SPX files are a rare find on computers today. There are three kinds of files that may use the .SPX file extension. Audio files using the Speex audio compression format may use the .SPX extension. Another possibility is that it could be an old file associated with Microsoft Visual Fox Pro, used by software developers. If you are using a Mac computer, it could be a OS X system report file that is opened only with the Apple System Profiler.

https://www.techwalla.com/articles/what-is-an-spx-file

If it's a Mac System Report, open it with Preview & Export as a PDF.

open with text edit

and choose

File>Export as PDF...