Error Opening Pre-2008 PageMaker Files Due to Plug-in Issues

Your only real option if you no longer have a system running CS6 (or earlier):

If you had CS6 previously and you had registered it to your adobe account, you should still be able to download the installer for it from there. This is assuming you have a compatible system to install it on (particularly on the Mac side). You may not be able to fully activate it, but even in trial mode you can convert as many files as you need during the trial period. Plus I don't recommend installing it alongside any newer CC version as it messes up file associations (particularly on the Win side)

Note: the previously posted tip of downloading the CS6 trial installer (from prodesigntools website), no longer works as Adobe has disabled the redirection of these download links.

Otherwise, you can also appeal to some here that have the ability to do conversions for you.

You can only open PageMaker files in InDesign CS6 or earlier.

https://helpx.adobe.com/indesign/kb/cant-open-pagemaker-files-indesign.html

~Barb