The man page for xargs says:
-I replace-str
Replace occurrences of replace-str in the initial-arguments with names read from standard input. Also, unquoted blanks do not terminate input items; instead the separator is the newline character. Implies -x and -L 1.
In other words, you're ending up with a single argument called "20170105170516234.pdf 20170105173126944.pdf 20170105173209758.pdf 20170621163418079.pdf
"
I suggest ditching xargs altogether and re-ordering your command something like this:
pdftk $(exiftool -p exifprintformat -if '$Subject =~/DATA/i' -q *.pdf |grep pdf |sed 's/ //g'| tr '\n' ' ') cat output binder1.pdf
This all assumes you don't have spaces in your filenames (safe assumption since you were removing all spaces with sed anyway).