https://stackoverflow.com/questions/17503349
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Which version of the PDF specification are you reading? In the version I have here (PDF 32000-1:2008) the "/DA" key at the location you mention (under the AcroForm key) is optional, not required.
The key is required only in those fields who have variable text (i.e. text not known in advance such as (example of the PDF specification) fields that are filled in by a user.
The PDF specification is also quite specific what this key should contain. Its value should be "a sequence of valid page-content graphics or text state operators that define such properties as the field’s text size and colour."
That is exactly what you found in your example - the string "/Helv 0 Tf 0 g" is a sequence of operators as they would be used on a page to specify a font and drawing color. More specifically:
/Helv 0 Tf, indicates that a font should be used that is called "/Helv" in the resource dictionary (so "/Helv" is the name under which the font is known in a resource dict, not necessarily / likely the actual font name) and that it should be used at 0 point size (which indicates an autosize font that automatically adjusts so no scrolling in the field is required - see also helpful comments). Tf is defined in paragraph 9.3.1 - Text State Parameters / General - in my version of the document.
0 g, indicates that text should be colored using grayscale black - it is defined in 8.6.4.2 - DeviceGray Colour Space.
As to your question on "what will happen when I omit these"... it is a VERY bad idea (allow me to repeat that - VERY bad idea) to omit keys that are required. At best you'll get some default behavior from some viewers. If the specification says something is required, make sure you understand what it is and why it is there and follow the specification.
Please. You have no idea how much trouble bad PDF files cause in all sorts of different workflows.
