I developed another tool pdfredactoronline.com that does this. Full redaction/removal of content, not just blacking out text. I didn't include it in this tool for now due to the APGL license of the library that enables the redaction, but since something fully in the browser is kinda open source already, I'll see if I can comply with the license while including it in BreezePDF

LibreOffice.

You can open PDF in LibreOffice Writer, just make sure to select "PDF - Portable Document Format (writer)" in the file selection dialog. This is very important, if you don't do that, the file will be opened in LibreOffice Draw instead of Writer.

Do you want the black rectangle and the content removed? There are "redaction" tools. https://smallpdf.com/redact-pdf

This likely uploads your content to their servers

you can do it easily with Inkscape which supports the pdf format

Honestly, your best bet for securely erasing data from a PDF is by adding the black bars, printing it and then scanning the print.

You can bypass the messy printing step by just printing the PDF as an image, then import or OCR the image as a new document.