Outlook 2013 PDFMaker archiving skips most of the emails

The Office COM addin skips most of the selected emails to convert to PDF. It's almost as if it's timing out.

There are thousands of emails that I need to convert, but usually after about 20 or 30, it starts to mark items as "skipped" and status: "item failed". This happens even if I only select 100 at a time - and it takes about 20 minutes to get through 100 items, most of which fail. I get similar results when trying to do it by folder.

Any idea what the bottleneck is? This is Acrobat Pro DC 2015.006.30417 (the latest version) with Outlook 15.0.5023.1000 64-bit on Windows 10.

Thanks.