An expandable folder is provided which includes a front panel member, a rear panel member, and an expandable gusset which connects the panel members and forms the bottom and sides of the folder. Reinforcing means are provided upon critical or highly stressed locations to enhance the durability and useful life of the folder. Various types of reinforcement are provided about and, preferably, over the top of the gusset. The end portions of the gussets along the sides and, where applicable, the bottom, preferably have a width which is at least twice the width of the gusset folds. The outer surfaces of the inner front and rear panel members are also reinforced to strengthen the folder at least in the areas where the gusset is attached, and, for best results, on the entire outer surfaces of the panels.