Your regex is simple enough (i.e., it uses no regex construct introduced after Perl version 5.6) that the hoary YAPE::Regex::Explain can still be informative:

c:\@Work\Perl\monks>perl -wMstrict -le
"use YAPE::Regex::Explain;
 ;;
 print YAPE::Regex::Explain->new(qr/\.[docx|pdf]$/)->explain;
"
The regular expression:

(?-imsx:\.[docx|pdf]$)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  [docx|pdf]               any character of: 'd', 'o', 'c', 'x', '|',
                           'p', 'd', 'f'
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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See also perlretut, and perlrequick.