Handling a .txt text

[reinalazaro]

Thank you guys for the help!
I'm moving forward with your tips, but it breaks me another issue.

f = open("file.txt", "r")

for line in f:

    for item in line.split(' '):

        if "NET" in item.upper():

            print(line)

        else:
            pass

f.close()

When I open a .txt with more data the return of this code is the error below at PyCharm:

Error:
File "C:\Users\Lzr\AppData\Local\Programs\Python\Python37-32\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 4748: character maps to <undefined>

I'm wondering there is something wrong in the .txt file but there will be a way to treat this "decode" error.
Thank you again!!

PS.: I liked the way to open the file using 'with'.