Well, to answer the basic question, "CAN they view and steal documents," YES, of course they can.
How much effort it might be depends entirely on how much effort you've put into prevention, such as anti-virus software and not opening email attachments, no matter how tempting.
As to likelihood, well the good news and bad news is that you are no more likely to be hacked than the next guy, but also, no less likely.
Some hackers are very specific with their targets. There's no money in snatching your letter to Grandma. Others have a more "shotgun" approach. They just want to hack into as many systems as possible, either just for the fun of it, for the pride of showing off to their peers, or so they can hi-jack your computer for other nefarious ends.
Basically, just follow the regular rules for safe-surfing. Keep your anti-virus program running and up-to-date, don't open any email attachments, stay away from all "those" sites (you know which ones I'm talking about!), and you should be as safe as you need to be.
If you're really paranoid about your documents, since you mentioned that, download some free encryption software. PGP is good and I think still has a free version. I use "TrueCrypt". Keep your documents encrypted, and even if they do get stolen, the hacker won't be able to do anything with them. (Be careful too about the Temporary Files that Word opens!)