Need help with website that generates custom pdfs

I need some help. My website generates custom pdf documents based upon information the customer submits. Once finished, they can preview the pdf form and if they like it, can buy an non-watermarked version. The site keeps having error messages from time to time when people try to download their paid-for document. The message they get is usually: Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /hsphere/local/home/users/[login to view URL] on line 29 FPDF error: Some data has already been output, can't send PDF file Why is this? Is it a cache issue? The will work fine for months, order after order, and then all of a sudden it shows error messages when trying to download documents. I need the site to run entirely on its own and I can't afford these problems from time to time. Also, I would like to be able access documents that customers paid for, in the rare case they can't access them. This way I can email them to them later, rather than have to refund everybody. Right now, I have NO WAY to access old documents from customers. I would love in the admin panel, to be able to pull up their order#, click a button and regenerate a pdf of what they ordered document. That way I can download it and send it to the buyer. The cart was designed using zencart, I believe. I can send more details. I will send more details to the buyer. This needs to be fixed asap.