If you don't have installed program for editing PDF documents on your computer, then you can use web application PDFescape which allows you to edit your documents in your browser.
Google Chrome has a handy function where you can just paste an url or search term and you will go straight to the page or get the Google search results.
Remember as a kid you played with a mirror, putting your face to the side of it and your friends were laughing at the funny symmetry of your face and your faces reflection?
Let me show you how to achieve that Depth Of Field effect known from the photography using Gimp. With other words, how to make selected area of the photo sharp and the rest out of focus.
