an expandable folder is provided which includes a front panel member , a rear panel member , and a gusset member which connects peripheral portions of the bottoms of the front and rear panel members and at least a portion of the sides of the panel members to form the expandable folder . the gusset member includes a sheet having a plurality of accordion - like folds , with terminal folds of the sheet being wider than the other folds , and the terminal folds are connected to the front and rear panel members to enhance the connection therebetween . the expandable folder further comprises interior reinforcing strips positioned along at least part of the connection between the gusset member and panel members peripheral portion for preventing separation of the front and rear panel members from the gusset member when the folder is used . reinforcing strips may also be provided along the top or corners of the gusset member or on outer peripheral portions of the panel members . in addition , the outer surfaces of the panel members may be coated with a polymer to increase the resistance of the folder to water penetration and to protect the folder from disfigurement .