Sometimes a digital document appears differently on different computers and prints differently when different printers are used. This happens often with documents like forms and books that have a lot of formatting. Adobe developed the PDF file format to eliminate this problem. The PDF file format that allows a document to appear the same no matter which computer they are viewed on or which printer is used. This format was developed by Adobe as an open-file format so it is widely used. Adobe distributes a free software program called Adobe Reader which is the software needed to properly read PDF files. PDF files and the Adobe Reader program can be used on both PCs and Macs.