I have attached Screenshots from the Flow and Shape Details. I'm not using any Custom Section to display Attachment, I'm assuming the Attachment would show up and available to download in the Right navigation under attachment section (OOTB).
It looks like you are using the pyCaseAttachments section. I tried the version in Pega-EndUserUI:07-10-17 and UI-Kit-7 and both are working for me. The pyAttachments property is used in those sections and it should refer to D_AttachmentList. Your screen shot looks a little different from what I see. It looks like yours shows the category; whereas, I just see the file name.
This was my test.
Can you set up a simple test using the OOTB Administrator@pega.com operator and see if that works? If it does, then you can compare rules to see if there are differences in your application.
I created the same use case [added a Create PDF smart shape in the flow] on my Pega 7.4 system. It worked ok. I saw the PDF file was created. Can you check the tracer log and see if Work-.pxCreatePDF is invoked?