WPViewPDF 3 is really old and there are a lot of changes. I serviced that also for years.
You can test V4 - the demo is practically not restricted.
But here it seems so that you somehow unload the PDF while one is still printing.
Did you lock your load logic until the print command has been returned? This can be implemented easily with a booleen variable.
Checking the code in COMPDF_Print V4 sends messages to the calling window, this are
MSGPDF_PRINTSTART = 110; // Param = page count in the printing cue (not the total page count)
MSGPDF_PRINTPAGE = 111; // for each page, LParam = page number in document (see MSGPDF_PRINTPAGEPROGRESS)
MSGPDF_PRINTEND = 112; // done, LParam=0
MSGPDF_PRINTPAGEPROGRESS= 116; // Message to set prograsspar. param=position, max=param of MSGPDF_PRINTSTART