How do I find multiple strings in a text file?

I believe your approach would work, but it is very verbose and not very Pythonic. Try this out:

import os, subprocess

string1 = 'biscuit eater'

with open(os.path.expanduser("/users/acarroll55277/documents/Notes/new_myfile.txt"), 'r+') as fptr:

    matches = list()
    [matches.append(i) for i, line in enumerate(fptr.readlines()) if string1 in line.strip()]
    fptr.read().replace(string1, string1.title())

    if len(matches) == 0: print(f"string {string1} not found")

    [print(f"string {string1} found in line {i}") for i in matches]

This will now print out a message for every occurrence of your string in the file. In addition, the file is handled safely and closed automatically at the end of the script thanks to the with statement.