Many tools do not fully remove metadata, but just remove the link with in the metadata table. The data are thus still available in the PDF file itself.
While a lot of people rely on Exiftool to remove metadata, it actually does the same in PDFs. If you remove metadata with exiftool -all= some.pdf, you can always restore the data with exiftool -pdf-update:all= some.pdf.
There are several options to remove PDF metadata safely:
- Remove metadata with exiftool : 
exiftool -all= some.pdf - Then remove ununsed objects with qpdf : 
qpdf --linearize some.pdf - > some.cleaned.pdf 
