Sejda is a free online tool to add a watermark to PDF files. Its free version lets you upload a PDF file with up to 200 pages or 50 MB in size. Also, you can perform up to 3 tasks per hour.

Foxit supports tons of file formats, and adding hyperlinks, images, and files to a PDF is trouble-free. Indeed, ease-of-use is one of Foxit’s great strengths.