What Feng Chentan said is basically correct. But you can modify it and save it.

In the document window:

Execute tools/options/security commands;

Enter the password in "Password when modifying file";

Click the "Document Protection" button to set the document protection authority;

Click the "Yes, start compulsory protection" button, enter the password and confirm.

Effect:

Open the document, you can only browse it, but you can't modify it.

Make changes to the document:

Execute the Tools/Unprotect Document command and enter the password to confirm.