In a help system, you likely have bookmarks and such, other wise printing to a new PDF with expanding to fit would be a simple approach. Actually it is probably best to go back to the original and change paper size, making a new PDF.
Other options include using the crop tool to change the paper size. Unfortunately, the body does not generally expand to match the text. It may be that just printing to a new PDF would be the simple root to go since you are talking about a print version, not a linked version. Be sure to select the paper in the printer properties and then select print to printer margins. This will give a large print version. If you want to keep the smaller printer size, you really need to go back and deal with the original and make a new PDF. If you just make a copy of the original, you would have both versions available.
If you want to keep the regular font size, yes. PDFs can not be edited in the sense I think you want. If you simply want to zoom the page out to 8.5X11, then you can print to a new PDF and adjust the paper size and Fit to Margins. Since the purpose is for printing, there should be no concern about bookmarks and such. You can do this with some regular printer drivers also, but you were wanting a PDF of 8.5X11.
My guess is that letting WORD reflow to a new layout will give you more of what you want. Just make some copies of your files and try a few options to see what works best for you.