v4.0.0

BREAKING CHANGE

• ⚠️ Switch from pdfjs to pdf-lib.
  If you want to continue useing pdfjs please stick with v3 release.
• ⚠️ The add function is now also async!
• This following issues should be fixed now:
  • #42 Merge Compressed PDF gives error
  • #64 Merge Pdf not merging again with other pdf
  • #31 Merged result PDF file is 3x bigger than expected