Encryption of exe file
The term "plaintext" in cryptography does not imply that the input is actual text. Traditionally, the plaintext was often actual text, but the term just refers to the input (other than the key) to the encryption algorithm. Plaintext can be a sequence of letters, like in many classical ciphers; it can be an analog telephone signal; it can be an image or map; it can be whatever the cipher uses as an input. In modern digital ciphers, the plaintext is a stream of bits, possibly with length constraints, because that's the kind of data you already have when you're working with computers. Modern ciphers can handle any stream of bits (again, possibly within those length constraints); it doesn't matter what the stream of bits represents. An EXE file is a stream of bits, so it can be encrypted.