It the PDF is a true PDF Form then there will be form fields present for entering the requisite information.
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If the PDF page content has the appearance of a "form" but there are no form fields then you do not have a PDF form (a digital form if you will).
Such would be used to print to hardcopy and then completed with pen and ink.
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However, if there are not form fields and you used Adobe Reader XI you could use the "Add Text Comment" (formally the "Typewriter" tool) to add text.
Use Comment :: Annotations :: Add Text Comment (the capital "T")
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Be well...