I think your are using a Log Message activity. The characters are limited when you use Log Message activity. You should have all lines in your string variable. It is just the visual output of the variable is limited to a certain number of characters in UiPath. In short, your string variable has all the lines, not to worry.
This is also similar in any other programming language. The reason for this is to give the user a taste of the output and not the entire output. For example if someone tried to Log 100000 lines, the debugger will have to both process and display 100000 lines. So they limit characters in terminal / console outputs.
To check if all lines exist:
Approach 1. Try using Write to Text File activity and pass your string variable as text value. Provide a dummy text file name “TestingOutputs.txt” → Open text file and you should see the same text as your other input file.
Approach 2. Use a Message box activity and use the following YourInputString.Split(Environment.NewLine.TocharArray).Count
YourInputString ( The one you have already read from the textfile). You should see the same line counts (123 in this case)