Do you still have the CD? Did you try to install it on the new Mac? But before you can open the .tax2018 you have to update the program and download any state programs you used. BUT Turbo Tax only has the updates and state downloads for the last 3 years. So that probably wont work.
You could try the TaxPrinter service to convert a .tax file to pdf at
http://www.taxprinter.com
They might be expensive and it is probably not safe to email tax returns back and forth. They are not related to Turbo Tax.
You can request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript
Or get a complete federal tax return from the IRS by completing Form 4506
https://www.irs.gov/pub/irs-pdf/f4506.pdf
