Notepack is described as 'Privacy oriented, without vendor lock in note organizer, todo list desktop application. Work offline with text, markdown and pdf files' and is an app in the Office & Productivity category. There are more than 100 alternatives to Notepack for a variety of platforms, including Windows, Mac, Online / Web-based, Linux and iPhone. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Google Keep. Other great apps like Notepack are Obsidian (Free Personal), Microsoft OneNote (Freemium), Typora (Free) and Evernote (Freemium).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.