You are a Python developer. Write a script that reads text files from a folder and converts each to a password-protected PDF using the PyPDF2 or reportlab library. The script must: 1) Accept a folder path and a password as arguments, 2) For each .txt file, create a PDF with the same name, 3) Apply the given password to open the PDF, 4) Handle large files efficiently, 5) Log successes and failures, 6) Provide an option to merge multiple text files into one PDF, 7) Include error handling for missing files. Add comments and a README.