Similar to this question , but the solution of moving files to the home directory didn't sort it for me. So I don't think its a problem with permissions.
This is the jist of the error I got:
Error: Unable to find file.
Error: Failed to open PDF file:
18.png
Errors encountered. No output created.
Done. Input errors, so no output created.
And the command entered:
pdftk 1.png 2.png 3.png 4.png 5.png 6.png 7.png 8.png 9.png 10.png 11.png 12.png 13.png 14.png 15.png 16.png 17.png 18.png cat output trippyblsmq.pdf