The term "pdf" stands for Portable Document Format. This format was created by Adobe Systems in the mid-1990's for the exchange of digital files. On July 1, 2008, Adobe Systems officially released the "pdf" format for general non-propriety usage. Because of its widespread use, most word-processing, graphic, and spreadsheet software have built-in publishing routines to create pdf's.
To maintain legibility of uploaded documents, we recommend that resolution not be dropped below 600 dpi on drawings and not below 300 dpi on text documents.
Other than vector (i.e. CAD) files, all documents are to be submitted (a.k.a. uploaded) using the pdf format. This includes letters, reports, plan drawings, images, and all other supporting documentation.