getPageNthWord returning [object Doc] when used in a text field custom calculation

Since you've mentioned  about a bit of context, I think you're not using that function in the correct context.

I am learning JavaScript so bear with me if I say something incorrect, but you can't convert a function to string if the object itself is not defined to begin with.

Running the function by itself in the console is not converting anything to a string, is just reporting back. 

So when you say "When I try to the use the function this.getPageNthWord() or doc.getPageNthWord() and convert it to a string" is hard to make sense out of it, unless you mean that you have defined an array for example, or a for loop, with  a condition to search for a specific word. And since you've mentioned about using the function by itself, and that it finds the first word in the document, that is not a text string that was taken from a field object for example. The text string that you're referring to is taken from the free text content of your document. That doesn't necessarily mean it was found in an annotation or a comment.

See Example 2, in Page 232 of the Adobe Acrobat SDK JavaScript API JavaScript™ for Acrobat® API Reference "Doc methods"

//Example 2
//Search through the document for the word “Acrobat” and create a link around that word.

for (var p = 0; p < this.numPages; p++)
{
var numWords = this.getPageNumWords(p);
for (var i=0; i<numWords; i++)
{
var ckWord = this.getPageNthWord(p, i, true);
if ( ckWord == "Acrobat")
{
var q = this.getPageNthWordQuads(p, i);
// Convert quads in default user space to rotated
// User space used by Links.
m = (new Matrix2D).fromRotated(this,p);
mInv = m.invert()
r = mInv.transform(q)
r=r.toString()
r = r.split(",");
l = addLink(p, [r[4], r[5], r[2], r[3]]);
l.borderColor = color.red;
l.borderWidth = 1;
l.setAction("this.getURL('http://www.example.com/')");
}
}
}

Post your full code, please.