Before I answer the question, I want to stress the importance of reviewing EDF contents before analyzing data stored in an EDF file.
The best way to review an EDF's content is to use one of the many publicly available EDF viewers. The EDF specification page lists publicly available EDF tools. We have made a MATLAB based EDF viewer available. Since most research is collaborative, I would suggest that working groups use the same viewer.
We develop a MATLAB program specifically for reviewing the contents of an EDF file. The program, SignalRasterViewer, is a general purpose utility for checking and viewing the contents of an EDF file.
The NSRR now lets you preview EDFs in your browser using the new Altamira EDF viewer.
Once you have submitted an agreement for a dataset, you will be able to see a link to preview the EDFs in your browser as seen in the screenshot below.
Once you click the link, you will see the EDF, and will be able to navigate forwards and backwards through the EDF. Please note that EDFs with signals with high sampling rates may load slower than other EDFs.
As always, for more in depth analysis, we recommend to use the downloadable EDF viewers that @DennisDean mentioned above.