Most of the editable documents available from this website are created or edited with Libre Office (Word equivalent, free)
“LibreOffice is compatible with a wide range of document formats such as Microsoft® Word (.doc, .docx), Excel (.xls, .xlsx), PowerPoint (.ppt, .pptx) and Publisher. But LibreOffice goes much further with its native support for a modern and open standard, the Open Document Format (ODF). With LibreOffice, you have maximum control over your data and content – and you can export your work in many different formats including PDF.”
About LibreOffice https://www.libreoffice.org/discover/libreoffice/
You can download a copy from:
https://www.libreoffice.org/download/download-libreoffice/