SmartRM is a free tool which allows you to share PDF and image files with others, yet still keep some control over how they're used. You could prevent them from being printed or shared with others, for instance; set time limits on how long they can be viewed, and more.
The program first requires you to create an account with your name, email address and some logon details. This is all very easy, and handled from within SmartRM.
The program can then protect any PDF or JPG in various ways. You'll need to enter the email addresses of everyone you want to be able to view it, for instance. You can decide when the file will be accessible (for the next 7 days only, say, or within two dates and times which you define). Other options allow you to control the number of times it can be viewed, whether it can be printed, and even if it can be viewed offline.
Once this has all been set up, SmartRM produces an encrypted version of the file. Pass this on to the people on your distribution list, they'll have to download and install SmartRM themselves, and it will display the document - but with all the restrictions you've defined.
There are obvious limits here. You can't stop people taking screen grabs of your images or documents, for instance. And even if the file stays on a recipient's system, there's no way to control who else might view it there.
But, there's also no doubt that SmartRM makes sharing much more difficult. A recipient can't just pass on the original file to someone else. And the ability to set an expiry date also gives you some useful control. Give them a three hour window to view a 100 page document, say, and with any luck they'll be too busy reading it to try and thinking about hacking the system, or finding a way to create a local copy.