Fortunately, the Adobe Reader app for Android phones (maybe for iPhones, but I don't know since I'm Android all the way) makes this exact scenario a thing of the past. With the Adobe Reader, you can sign your .pdf documents easily and free of charge from your Android powered devices. Today, I'm going to walk you through how to do that.
First, make sure you have the Adobe Reader app on your device. Then, open up your .pdf file in Adobe.
Now, tap in the middle of your document, which will change the bar at the top to look like the following:
Choose the pen (the last tool on the right), and you'll receive a toast message telling you to tap on the screen at the location you want to insert your signature. I'd choose right on the signature line.
Once you've tapped where you want your signature to go, you'll be taken to a screen where you will be asked to sign your name. Simply do that. Sign your name. You can modify the look of it with the controls at the top. When you're done, tap the check mark.
After a brief dialog asking you to give your signature a name, you will be taken back to your document, but it will have your signature added in now on the signature line.
If you want to modify your signature's appearance on the page, simply tap on it. You will be given options to modify how it looks as well as to re-size it and move it to a different place on the screen.
You now have a saved document with your signature on it, ready to return by email, print, or whatever else you want to do with it. Even better, when you go to sign something else, it will remember your signature and give you the option of simply inserting it.
So, why would you ever use this? Well, for starters, it eliminates the scenario described at the top. IT saves trees. Moreover, if you have a document you want a client to sign, you can have them just sign your phone.