PDF join files and replace strings using pdftk

pdftk *.pdf cat output combined.pdf

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You can try to modify content of your PDF as follows

1. Uncompress the text streams of PDF

   pdftk file.pdf output uncompressed.pdf uncompress

2. Use sed to replace your text with another

   sed -e "s/ORIGINALSTRING/NEWSTRING/g" <uncompressed.pdf >modified.pdf

3. If this attempt was successful, re-compress the PDF with pdftk

   pdftk modified.pdf output recompressed.pdf compress

Sources:
https://www.pdflabs.com/docs/pdftk-cli-examples/
http://stackoverflow.com/a/9872494/4151875