For personal organization and project management, choose Obsidian. Its robust customization options and local storage cater well to individuals who value privacy and prefer working offline. While it might have a learning curve, the Markdown support makes it ideal for users who want to craft detailed notes without cloud reliance. Avoid this if you need real-time collaboration or can't navigate its initial complexities.
If your focus is on handling high volumes of PDF documents securely, go for Stirling-PDF. Its open-source nature and high-level security make it suitable for businesses that prioritize document safety and need to process many files. However, be aware that the paid model can become costly with heavy usage, and the limited mobile functionality may not suit those who require robust mobile PDF capabilities. Opt for this if security and volume are your top concerns over initial cost or mobile usability.