Interesting. Here is a benchmark, to complete the picture:

hyperfine --runs 10000 'gzip -k -c a.pdf -c b.pdf c.pdf > out.gz' 'tar
+ cf out.tar a.pdf b.pdf c.pdf'
# truncated output
 
Summary
  'tar cf out.tar a.pdf b.pdf c.pdf' ran
    1.72 ± 0.57 times faster than 'gzip -k -c a.pdf -c b.pdf c.pdf > o
+ut.gz'
[download]

Possibly too few files but tar is significantly faster.