Convert PDF to ZPL, Fix text side-effect update on cell

[johnfercher]

Description

• Convert PDF to ZPL
• Fix text side-effect update on cell

Related Issue

Checklist

check with "x", if applied to your change

• All methods associated with structs has func (s *struct) method() {} name style.
• Wrote unit tests for new/changed features.
• Updated docs/doc.go
• Updated pkg/pdf/example_test.go
• Updated README.md
• Updated all examples inside internal/examples
• New public methods/structs/interfaces has comments upside them explaining they responsibilities
• Executed make dod with none issues pointed out by golangci-lint and goreportcard-cli

[codecov]

Codecov Report

All modified lines are covered by tests ✅

❗ No coverage uploaded for pull request base (v2@c2a8ed2). Click here to learn what that means.

Additional details and impacted files

@@          Coverage Diff          @@
##             v2     #315   +/-   ##
=====================================
  Coverage      ?   26.00%           
=====================================
  Files         ?       20           
  Lines         ?      958           
  Branches      ?        0           
=====================================
  Hits          ?      249           
  Misses        ?      703           
  Partials      ?        6           

Flag	Coverage Δ
unittests	26.00% <0.00%> (?)

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