One of the fun things about computers is playing with programs like Paint. You can use the tools in Paint to add something to a different document. The Paint program can help you make new image files, but it cannot open document or PDF files. While it is impossible to open a PDF file in the Paint program, there are several ways to paint a PDF file.
Convert your file to a PDF. Many programs can convert a PDF to a Paint-acceptable format such as a JPEG. Programs such as Preview allow you to export PDF files as pictures. You can also use a free file converter online, such as http://www.zamzar.com/.
Open your file in paint. Colour or paint the file in any way you desire.
Save your finished creation in whatever image type you prefer. JPEG is the most common image type.
Convert your JPEG image to a PDF file using the same program you used in the original conversion.
Find a computer with a copy of the Photoshop software. Many computer labs at schools have this software.
Open the PDF in Photoshop and add a "layer" to your file. Add any embellishments you desire on the new layer. This allows you to delete new additions without altering the original image. Photoshop allows you to play with a large variety of colours, brush types and interesting effects.
Save your file in its original format.
Sign up for a free trial of Photoshop to experiment with the software.
Don't forget to save a copy of the original file and to rename your painted version.