Once most documents (PDF, Word document, Excel document, Power Point presentation, etc) have been uploaded they cannot be edited from within Typo3.  Edit the original on your computer then re-upload the document.

There are exceptions.  If the document was created in Typo3, you can edit it in Typo3.  The file types that can be created are: [txt, html, htm, css, tmpl, js, sql, xml, csv, php, php3, php4, php5, php6, phpsh, inc].   Most of these, however, are outside the scope of most sites.  A text file (.txt) might be the exception.  You might find yourself editing a text file.

To edit a file in Typo3, find the file in your folder(s).

Left click on the small file icon to get the file's contextual menu.  Pull down to "Edit".

The contents of the file will be pulled into a plain text editor.  NOTE: this is not a rich text editor.  

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