The most likely reason is that your document contains Unicode or UTF-8 text that NoteTab could not fully convert to the selected code page. When NoteTab opens Unicode files, it has to convert them to the ANSI character set (system default or code page specified in the File Open dialog), which supports much fewer characters. As a result, the conversion process may drop non-ANSI characters resulting in loss of information. When the Protect Unicode Files setting is checked (General tab in Options dialog box), Unicode files with characters that failed to convert are opened in Read-Only mode, which protects the document from changes.
If you know that your Unicode or UTF-8 documents will not lose important information during the conversion process, uncheck this option so that such files are always editable. You can also make a document editable by unchecking the Read-Only option under the Document menu.
To make Unicode files editable, go to View > Options > General, then uncheck Protect Unicode Files and click OK