Ask the instructor what word processor they use. You may be able to save your documents in the file format of the word processor that your instructor has. If you have to save a document in text format, you will have to enter a hard return at the end of each typed line by pressing the enter key. The reason why a file cannot be opened is because your computer does not know what format the file is. In a case such as this, you would want to send the person a message asking them what format the file was saved in.
Most software packages have the ability to open many different types of files.
Once you know what the format is, open your software package. When your software package tries to open the file and doesn't recognize it, you will be prompted to indicate what format the file is. More times than not, your software package will be able to open the file.