Controlling a dc motor with a saved text file, is this possible?

Yes you can - if the text is already known at coding time

you can store your strings if they don't change in program space - Check PROGMEM

here is a bit of code to study

char message1[] = "Text in SRAM, can change: ABCDEFGH";
const char message2[] PROGMEM = "Text in flash memory, locked: ABCDEFGH";

void setup() {
  unsigned int message1Length, message2Length;

  message1Length = strlen(message1);
  message2Length = strlen_P(message2); // The strlen_P() function is similar to strlen(), with a pointer to a string in program space

  Serial.begin(115200);
  Serial.println("Displaying ASCII CHAR");
  Serial.println("****************");
  Serial.print("message1=[");
  for (int i = 0; i < message1Length; i++) Serial.print(message1[i]); // print each char
  Serial.println("]");

  Serial.print("message2=[");
  for (int i = 0; i < message2Length; i++) Serial.print((char) pgm_read_byte_near(message2 + i)); // print each byte as char
  Serial.println("]");


  Serial.println("\n\nDisplaying ASCII CODE");
  Serial.println("****************");
  Serial.print("message1=[");
  for (int i = 0; i < message1Length; i++) {
    Serial.print((byte) message1[i]); // print each char code
    Serial.print(" "); // print a space

  }
  Serial.println("]");

  Serial.print("message2=[");
  for (int i = 0; i < message2Length; i++) {
    Serial.print(pgm_read_byte_near(message2 + i)); // print each byte (as byte)
    Serial.print(" "); // print a space
  }
  Serial.println("]");
}

void loop() {}

There are limits to array sizes to what a 2 byte pointer can index and of course physical memory