I'm pretty sure I used pdf/x-4. What I cannot remember is whether I left the Use Advance Features checked or not.
Oh, one more thing. The document dpi should probably be 150 instead of the 300 it is. That will make rasterizing faster and for a document of this size is likely the resolution it will be printed at. But do check with your print service.
I successfully exported two PDFs with transparency intact. For the first one, I used your file as is. For the second one, I clipped the image into a rectangle the size of your artboard, then rasterized that. Then made a PDF.
In both cases, the export takes a huge amount of time. The rasterization probably took as long as exporting a PDF, but I walked away for each process and didn't time the processes. I do note that the titlebar shows AD as not reponding when rasterizing that image. But if one waits long enough AD finishes and all is fine.
I can upload the PDF and the modified AD file later if you wish...but I would be using my phone as a hotspot until later.
The PDF creation time needs seriously looked at by Serif.
In the case of the first PDF I created, while I was waiting, I downloaded a customer file to my laptop, made changes, exported a PDF, uploaded that 100 meg PDF, cooked some soup and made coffee before AD finished. Really. But I was using other software. Nothing I have takes the amount of time that AD takes...
I'll wait to hear back from you before I delete these files (nice looking poster, BTW).
