Files produced by word processors are usually not saved as plain text files. They contain all sorts of control codes that only the word processor that created them (or compatible programs) can understand. Word processors use those special control characters to store text formatting information (bold, italic, line spacing, paragraph alignment, etc.).
The NoteTab editor only understands plain text files (that’s why it is so fast compared to a word processor) and will display the special control characters as funny symbols or rectangles on the screen. Some of the control characters may be interpreted by NoteTab as signaling the end of a file. This explains why such files are rarely fully loaded in the editor.
Unless you are very confident, you should not edit or save such files with NoteTab. Doing this may render them unreadable to your word processor.