Hi,
V5
File print A4.
I had centred the window print on my subject, made a pdf print with Bullzip and it displayed as this in pdf viewer, however open pdf into pshop and now subject is upper left.
Carry out a test …
created 50 x 50mm square, file print pdf, scale 1:1 centre A4 window on box. hit enter and complete the command.
open resulting pdf in pdf viewer and box is in centre of A4 page. 
Open this into Photoshop and see just a 50x50mm image ! Box forms the image boundary.
repeat and have it also print grid, see A4 with grid across middle area, in pshop it opens with grid deciding the page size, not A4, but squarish.
The pdf will only display at its A4 size in photoshop if artwork fills the entire pdf.
see attached test files, how do they open into your photoshop ?
test bullzip positioning grid on.pdf (5.5 KB)
test bullzip positioning grid off.pdf (3.2 KB)
I have tried drawing an A4 portrait rectangle centred on the 50x50 box, this will put a thin line at edge of pdf, so forcing pshop to acknowledge the shape, but its impossible to get the A4 print window to lock to it, asks change scale, yet I am at 1:1 as needed, controls over this print box are very odd indeed.
Steve