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Hello,

The problem is that the PDF is corrupted. Although some PDF viewers can open it, the document has internal problems. If you check the document, you get this information:

Entity: line 9: parser error : PCDATA invalid Char value 23
  <pdf:Creator></pdf:Creator>
               ^
Entity: line 11: parser error : PCDATA invalid Char value 5
  <pdf:Title></pdf:Title>
             ^
Entity: line 12: parser error : PCDATA invalid Char value 3
  <pdf:Author>€Ö!</pdf:Author>
                    ^
Entity: line 13: parser error : PCDATA invalid Char value 11
  <pdf:Producer>Ë}
                  •ùƒ</pdf:Producer>
                   ^
Entity: line 13: parser error : PCDATA invalid Char value 3
  <pdf:Producer>Ë}
                  •ùƒ</pdf:Producer>
                    ^
Entity: line 13: parser error : PCDATA invalid Char value 1
  <pdf:Producer>Ë}
                  •ùƒ</pdf:Producer>
                            ^
Entity: line 13: parser error : PCDATA invalid Char value 3
  <pdf:Producer>Ë}
                  •ùƒ</pdf:Producer>
                             ^
Entity: line 23: parser error : PCDATA invalid Char value 5
    <rdf:li xml:lang='x-default'></rdf:li>
                                 ^
Entity: line 26: parser error : PCDATA invalid Char value 3
  <xap:Author>€Ö!</xap:Author>
                    ^
Entity: line 34: parser error : PCDATA invalid Char value 5
  <dc:title></dc:title>
            ^
Entity: line 35: parser error : PCDATA invalid Char value 3
  <dc:creator>€Ö!</dc:creator>

If you repair it:

pdftocairo -pdf Keightley_1963.pdf Keightley_1963_repaired.pdf

the watermark is added perfectly.

Regards.