Allowing for the customer to enter an amount is not an easy option. Remember your original question was about a payment button not a full blown ecommerce checkout solution.
I don't know a way using a PayPal button to give both fixed and custom options in the same button. You could price by bands like 0-100 words for $x, 101-200 words $y, etc.
Or you may be able to use the donation button and allow your customer to enter any amount. You may need to make your own button without Donation written on it. And it's likely that PayPal may call it a donation on their forms.
I don't know of a simple no cost way to know that the upload has been paid for. If your process involves some manual action on your part, it would be simple enough to match your PayPal payment records with the customer upload before you do the work on the document.