Problem scenario
You are using Windows, and you want to modify a PDF file without uploading it to a website. What can you do (without paying for an application)?
Possible Solution #1 (e.g., to black out sensitive content)
1. Install Firefox.
2. Open Firefox and then open the PDF in Firefox.
3. Hover over the pencil icon with a squiggly line.
4. Click it and adjust the thickness to your desire.
5. You can edit the document now.
6. Save it with a new name.