"Try before you buy" is just that - something you are allowed to do before you part with any money. Once you have paid then the transaction is usually finished.
If you want more certainty about a book purchase before you select the "buy" button, you could turn to any of the various communities on the social media platforms, and ask for experience/advise. There are Traveller groups on Facebook, Discord and Reddit. Or, you could look at the product previews on DriveThruRPG.
Those will help you make a more informed decision. Or sometimes YouTube have product reviews for RPG books. Or you could just be an "adventurer" and buy blind or back a Kickstarter publication. The Kickstarters are usually the best way to see an official video of a new publication, before you need to pledge any of your money. The drawback of the Kickstarter is that they only happen once in a product lifecycle, and then it is all over.
I am the other way round, I have a couple of Traveller books without the PDFs and I wished I owned both - but, it is not going to happen retrospectively.