No. The compressor is Ghostscript compiled to WebAssembly, served from this site and run inside your browser tab. The page's security policy blocks it from sending data anywhere. Once the page has loaded you can go offline and it still works.

Balanced suits email and sharing. Smallest suits reading on screen. Print keeps images sharp enough for paper. Prepress keeps colour profiles for professional printing.

PDFs made mostly of text and vector drawings are often already compact. Ghostscript rewrites the whole file, so the result can come out slightly larger. Keep the original in that case.

Not yet. Remove the password in your PDF reader first.

Yes. shrink_pdf.py in the source folder does the same job with PyMuPDF, for example: python shrink_pdf.py file.pdf --first-page