Add header/footer

Type {page} where you want it and {pages} for the page count, so "Page {page} of {pages}" comes out as Page 4 of 12 on the fourth page. {total} means the same as {pages}. There is also {date}, {time} and {filename}. Every other character is literal.

Yes. Fill the slots you want and leave the rest empty, or switch a whole row off. Leaving all six blank is refused with "give a header, a footer, or both", since there would be nothing to stamp. The header sits its own font size below the margin, and the footer sits the margin up from the bottom.

No. pdf-lib loads the file, draws the lines and saves it, all in a Web Worker in this tab. Nothing is transmitted, nothing is retained, and there is no account. You can watch the request counter stay at zero, or disconnect and stamp anyway.

The position, yes: six slots, left, centre and right along both edges, with a margin you set. Some pages, no. Every page in the document is stamped the same way. Appearance is the font size, the colour, and bold or regular, always in Helvetica.

You can, but they will overlap. Both default to 24 points from the bottom edge, so a centred footer here lands on a centred page number added there. Pick one: this tool already has {page} and {pages}, so a footer of "{page}" does what Add page numbers does.

One file per PDF you dropped, named with a -headed suffix, so report.pdf gives report-headed.pdf. Drop a whole batch and each is stamped the same way separately, with {pages} and {filename} resolved against each document of its own. Your originals are not modified.