Wondershare PDFelement - All-in-one PDF editor
Freeplane - Freeplane is a powerful and free software for building mind maps.
Foxit PhantomPDF - Edit PDF files with our feature-rich PDF Editor. Download Foxit PDF Editor to convert, sign, scan / OCR & more. A speedy PDF Editor alternative to Adobe Acrobat.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
OmniOutliner - OmniOutliner is a writing app for collecting information, outlining ideas, adding structure to any sort of writing.