Is there an easy way to encrypt a PDF?

You don't say whether you're running a Mac or Windows system, so I'll show you a solution for both. The Mac solution is quite a bit easier, but let's start with Windows because there's a nifty - free - application you can download to do just what you ask. I'm using Windows Vista for this demo, but the application works with both Vista and Windows XP.

The application I found to encrypt on the Windows side is called PDF ReDirect and you can download it from one of my favorite Windows download sites, download.com.

Once you have it installed, launch the app and drill down to the folder that contains the PDF file you want to encrypt. As you can see in the below screen shot, the interface is a bit confusing, unfortunately, once you're ready to encrypt the file:

I've labeled the areas in red to make this easier: you need to check "Encrypt PDF File" and enter a password on the middle-right box (#1), move your PDF file into the "merge list" in the middle (#2) and you'll want to rename the resultant file on the bottom box (#3) -- I added "-encrypted" to mine.

Once you have that all configured properly, click on "Save" near the bottom and you'll see:

A few seconds later, your file will be encrypted under its new name and you're ready to go.

The Mac Alternative

If you're running Mac OS X, however, you don't need to download anything and it's quite a bit easier. Open up your PDF file in "Preview" and choose File --> Save As.... You'll face the following dialog box:

Give the new file a different name to the existing file, click on "Encrypt", the click on "Save" and a second or two later your file is encrypted and ready to transmit.

Either way, it's always a good idea to keep PDF files encrypted on your laptop especially. Just in case someone else gets their hands on it...