Try:
byte[] data = File.ReadAllBytes(pathToFile);
If that gives you problems, then it may be that you need to check what else your code is doing, as an array can be up to 2GB. 750MB should be no problem at all.
If you can't load it as a complete item, then consider loading it as a FileStream - you can read that in "chunks" and most decryption methods have a method accepting streams.