Hi there largepdf,
How large is that PDF? There is a 100 MB file-size limit for uploading files to the ExportPDF service. Even if the file is smaller than that, its complexity are your Internet connection speed can also affect the upload/conversion. If the file is overly complex (for example, it has a large number of pages), then it's possible that the ExportPDF service is timing out.
If the PDF wasn't created from a scanned document, and doesn't require text recognition, you may have better luck converting with OCR disabled, as described in this document: How to disable Optical Character Recognition (O... | Adobe Community
Another option is to use Acrobat, which you can try for free for 30 days. Please see www.adobe.com/products/acrobat.html.
Best,
Sara