Nguyên the PDF Field Merger

fdf = Nguyen::Fdf.new(key: 'value', other_key: 'other value')
# use to_fdf if you just want the FDF data, without writing it to a file
puts fdf.to_fdf
# write fdf file
fdf.save_to('path/to/file.fdf')

xfdf = Nguyen::Xfdf.new(key: 'value', other_key: 'other value')
# use to_xfdf if you just want the XFDF data, without writing it to a file
puts xfdf.to_xfdf
# write xfdf file
xfdf.save_to('path/to/file.xfdf')

# adjust the pdftk path to suit your pdftk installation
pdftk = Nguyen::PdftkWrapper.new('/usr/local/bin/pdftk')

# find out the field names that are present in form.pdf
pdftk.get_field_names('path/to/form.pdf')

# take form.pdf, set the 'foo' field to 'bar' and save the document to myform.pdf

# with fdf
fdf = Nguyen::Fdf.new(foo: 'bar')
pdftk.fill_form('/path/to/form.pdf', 'myform.pdf', fdf)

# with xfdf
xfdf = Nguyen::Xfdf.new(foo: 'bar')
pdftk.fill_form('/path/to/form.pdf', 'myform.pdf', xfdf)

# will work with xfdf or fdf xml string directly as well
xfdf_string = <<XML
<?xml version="1.0" encoding="UTF-8"?>
<xfdf xmlns="http://ns.adobe.com/xfdf/" xml:space="preserve"><fields><field name="foo"><value>bar</value></field></fields></xfdf>
XML
# or xfdf_string = Nguyen::Xfdf.new(foo: 'bar').to_xfdf
pdftk.fill_form('/path/to/form', 'myform.pdf', xfdf_string)