Bob
Yes it is advisable to save the download to disk.
This brings me to the sensitive topic of copyright and further copying of the download. My copyright has what is called an angel policy which allows individuals who donate to make models from my design for their own purposes i.e. not for business or selling on.
To protect the design the download has four levels of visible protection and also has an embedded code that digitally links the individual download to the recipients email address.
Two of the visible layers can be seen by the naked eye and if a hacker wished they could remove them but the other visible aspects are so small they are hidden within full stops etc. The embedded code send me a message if the download is passed onto another computer (either by email or disk)
The reason for such onerous levels of protection is to prevent my designs appearing on places like ebay, because I can track the designs I can prove to ebay that copyright has been breached and they have to take action as per their terms and conditions.
None of this will effect 99% of people who download the designs.
Paul