CotEditor is a text editor designed exclusively for plain text. With CotEditor, you can open, create, and save a variety of files as plain text.
Plain text is text data composed only of characters and contains no style information, such as font, color, or embedded elements like images. In addition to .txt files, programming source files such as Swift or Python, and structured text formats such as HTML or Markdown, are also types of plain text that consist solely of character data.
CotEditor allows you to create, view, and edit plain text files like these. You can also open plain text files created in other apps. CotEditor can open any file as plain text.
Go to the CotEditor app 
on your Mac.
Do one of the following:
If the Open dialog appears: Click New Document in the bottom left.
If CotEditor is already open: Choose File > New.
When the document opens, begin typing or dictate text.
On your Mac, do any of the following:
Open from the Open dialog:
In the CotEditor app on your Mac, choose File > Open.
Select the document, then click Open.
If the document is stored in iCloud Drive, you can select CotEditor in the iCloud section of the sidebar, then double-click the document.
You can also specify the text encoding to use when opening a document. For details, see Open a document with a specified encoding.
Open by dragging and dropping:
In the Finder or another app, select the document you want to open, then drag it.
Drop the document onto the CotEditor icon in the Dock or another location.
Open by double-clicking in the Finder:
If the document’s file type is already associated with CotEditor in the Finder, you can open the document directly in CotEditor by double-clicking it in the Finder or another app.
Note: CotEditor also opens binary files that contain text data, such as PDF, RTF, or .plist files, also as plain text. To edit them in their native formats, use the corresponding apps.
In the CotEditor app on your Mac, open a folder to open it as a folder document. In a folder document, the folder’s contents appear in the file browser in the sidebar, and you can open and switch between documents in the folder within the same window.