I do not know if there is a solution. Photoshop does not have full support for Adobe vector file formats like ESP, AI, SVG, PDF, etc. However some Images saved by Photoshop as PDF seem to be save as files Photoshop supports. Photoshop File Open will open those PDF files as Photoshop layered document. However normally Vector files can not be opened by Photoshop File open. Photoshop Open will switch to vector file importers for Adobe file fomats that can render a single pixel layer for the content in the Vector file. There will be no vector paths in photoshop that relate to any of the pixel layer's content created. Vector Graphics is used during the import process the layer quality reflects the for any size you import the file the layers contents look good. It does not surprise me that Load Files into a Stack does not work file types Photoshop does not have support for.