"And another 38 percent either store tax documents on their computer's hard drive or in the cloud, approaches that are susceptible to a variety of hacks." - really? So, other than a local storage ("hard drive") or remote storage ("cloud"), what other approaches are there to storing documents that are not susceptible to any hacks? Paper printouts?
Encrypt your hard drive, choose good passwords for your cloud storage and don't share them with others. Your data is at most risk at your doctors office, btw (where they have all of your personal information, along with SSN and family records). I had 2 notifications of personal information theft from doctor's billing processor's offices in the last 2 years.