o Open the attached multi-page PDF file;
o Check that the four pages can be viewed;
o Resave the file as a PDF;
o Open the resaved file --> The opened file only contains the first page...

My recollection from long ago [probably using XnView Classic] is that the images in a multi-page PDF or TIFF file could be edited, but to avoid edits to an image being lost, it was necessary to resave the file before stepping to the next image, which could become slow as the number of pages increased.
My mistake or a bug?