Is there a letter preview w/o activity generation
This is a new feature in 4.7 which is undergoing beta testing at the moment.

In the meantime, as a work around I choose myself as the initial target for a new pdf letter and generate as many letters as I want until the content is correct.  It is then easy to search for those activities and delete them to tidy up my record.

If I were generating a letter where I needed to include a token that wasn't applicable to me, I would choose just one person on the list and take the same approach of deleting the "test" activities before creating the real letter. 

If the people wanting to create the letters don't have permissions to delete activities, then I instruct them to include "test" at the start of the subject whilst they are still editing the letter.  That makes it easy for me to delete the appropriate activities when they ask me to.