The RTF format allows you to open documents in any text editor. The RTF format was created by Microsoft to avoid converting a text document to another format (in order to open it in another program).
Steps
Method 1 of 2: Saving a New Document in RTF Format

Step 1. Launch a text editor such as MS Word (Microsoft), Apple Pages (Mac), or OpenOffice (free)

Step 2. Create a document (enter text)

Step 3. In the program menu, click "File" - "Save As"

Step 4. In the "File name" line, enter a name for the document

Step 5. Open the "Save as type" menu and select "Rich Text Format (RTF)"
Click "Save" and the document will be saved in RTF format.
Method 2 of 2: Saving an existing document in RTF format

Step 1. Double click on the document file you want to save in RTF
This will open it in a text editor such as MS Word (Microsoft), Apple Pages (Mac), or OpenOffice (free).

Step 2. In the program menu, click "File" - "Save As"

Step 3. Rename the document (if you like)
To do this, in the "File name" line, enter a new name for the document.
Even if you do not rename the document, the original document will not be overwritten, since the new document is in a different format (so you will get two documents - the original and the new one in RTF format). But if you have opened an RTF document, rename it to avoid overwriting it

Step 4. Open the "Save as type" menu and select "Rich Text Format (RTF)"
Click "Save" and the document will be saved in RTF format.
Advice
- Almost all existing text editors (regardless of operating system) support RTF format.
- Since RTF is a universal format, any features (for example, a certain formatting style) inherent in a given text editor (for example, Word) will not be saved in an RTF document.