Some of the documents in the "History of Frequency Control" are PDF files, so the Acrobat Reader (or compatible software) is needed for reading them. The Reader can be downloaded by clicking on the Get Acrobat Reader icon. Please note that you have to configure your browser so that it will automatically recognize and open a PDF file. If your browser will not permit such a "plug-in," then save the history paper to disk, open the Acrobat Reader, and open the history paper from within the Acrobat Reader.