Hi @Anonymous,
You can replace the text between the single quotation marks after the querytext parameter.
If you just want to look for that phrase in all file types that would be:
_api/search/query?querytext='Communication Strategy'&selectproperties='Title,Author'
If you are looking for a Word document with that phrase it would be:
_api/search/query?querytext='Communication Strategy filetype:docx'&selectproperties='Title,Author'