There are two layouts to the PDF files: non-linear (not "optimized") and linear ("optimized"). Non-linear PDF files can be smaller than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file. Linear PDF files (also called "optimized" or "web optimized" PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since they are generated in a linear (as in page order) fashion. PDF files may be optimized using Adobe Acrobat software or QPDF.