Hi Tom thanks for posting in the Community! Yes if you share your document with a professional or below level user they will be able to open it. However if you have advanced shapes on the canvas or you are using pro-level features that basic and below users do not have access to they may face restrictions in their ability to edit the document. Be sure to check out our Account Types article from our Help Center for more information.
Please note all free users have access to free 7-day trials that they can enable in their Account Settings. They can cancel at anytime. Hope this helps! Happy Diagramming!