Remove PDF.js mention from README.md

[voy]

Fixes #737.

[codecov-io]

Codecov Report

Merging #738 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #738   +/-   ##
=======================================
  Coverage   96.65%   96.65%           
=======================================
  Files          33       33           
  Lines        1227     1227           
=======================================
  Hits         1186     1186           
  Misses         41       41

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