The user manual is provided as a PDF document within the program .jar file.
Use the Extract Manual item in the Help menu to extract the file to your disc.
This will also attempt to open it in your computer's PDF reader program, e.g. Adobe Acrobat.
If the file fails to extract, e.g. because the program cannot write to your disc, you can
use a ZIP-file extractor such as ZIP or 7ZIP to extract the manual PDF from the .jar file.
You could also rename a copy of the .jar file to .zip and extract or copy the user manual from this.
If the PDF file cannot be opened, you probably do not have a PDF reader program, or
the file extension .pdf is not associated with the reader program.
Adobe Acrobat can be obtained here: https://www.adobe.com/.
7ZIP can be obtained here: https://www.7-zip.org/.