I have just downloaded long PDF document which shows as only covering half the page from left to right. I could create full pages by using return a thousand or so times but I wonder if there is a simpler way to fix my problem.

First, converted documents are ALWAYS a pain to edit. My preference is to paste the text as unformatted text in a new document and do my own formatting.
See Cleaning Up Pasted Text by Suzanne Barnhill, MVP

Lastly, do NOT use the Enter key for vertical spacing in a document.
2.2 Why you should press Enter only once to end a paragraph

I'm not sure what the point is of pasting as unformatted text or sending people elsewhere when there's a perfectly good solution at the top of this forum: https://www.msofficeforums.com/word/...-pdfs-etc.html

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

My memory is not as encyclopedic as yours, Paul. I remembered that I had a bookmark for Suzanne's page. Now I have one for yours as well. Suzanne's article takes people through a process manually, yours gives them a macro, a helpful macro.

As for pasting a unformatted or plain text, pdf conversions often contain numerous section breaks, web text often contains tables and frames that are unhelpful in the Word document.

All the more reason not to paste as unformatted text. Those 'unwanted' features at least preserve the basic appearance of the source and usually can easily-enough be cleaned up afterwards. Moreover, when pasting as unformatted text, hyperlinks, images and content in frames etc. are all lost.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

You can use your method, I will stick with mine. I am aiming for a document that can easily be edited in Word. My method gives me that.

I still have the pdf/web page to look at and can usually match the formatting easily without those frames and section breaks. I can copy and paste images back in.