Fix appender AcroForm loading

[adrg]

No description provided.

[codecov]

Codecov Report

Merging #355 into v3 will increase coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##               v3     #355      +/-   ##
==========================================
+ Coverage   55.21%   55.21%   +<.01%     
==========================================
  Files         152      152              
  Lines       27294    27299       +5     
==========================================
+ Hits        15070    15073       +3     
- Misses      10371    10372       +1     
- Partials     1853     1854       +1

Impacted Files	Coverage Δ
pdf/model/appender.go	75.93% <75%> (-0.21%)	⬇️

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