Most NetCDF files are in 2 formats and the suffix is an indication of what type it is. If the NetCDF file has . nc then it is binary. If the NetCDF file has .cdl then it is ASCII. Most browsers have options to download files in either binary or ASCII. You may also have to check your browser setting in MIME types as this can sometimes force a downloading to an unwanted file type. If you downloaded a .cdl file and the suffix is .txt then you should be able to just rename the file from .txt to .cdl. Then you should be able to open this file in wordpad or notepad. If you cant then it most likely is a binary file and you will have to change the way your browser downloads the file. If you want to use a NetCDF file in ArcMap you need to check if your NetCDF file has geo information. Not all NetCDF files have geographic information. Once you validate your NetCDF files does have geo information then it may work in ArcMap. Sometimes you may have to convert your NetCDF file to something ArcMap understands. For this i recommend using gdal tools from : https://gdal.org/ . Gdal tools will convert your NetCDF to a raster file (geoTIFF) or Shapefile or to KML. ArcMap depending on your ArcMap version may not read NetCDF files but will read geoTiff, Shapefiles, and KML. Hope this helps.

Reuben Reyes Thank you very much for your answer. It is just what I needed.

I quite agree with Reuben, thanks a lot. Good luck.