Image and PDF are two different kinds of file format, but with DpiApp, you can easily generate a PDF file containing all your images of a gallery.
First, go to the gallery you want to generate a PDF with. Then click the 'gear' button to switch to the 'Gallery Settings' tab, you can then toggle the 'PDF Download' item to 'On', and your PDF is ready!
Downloading PDF is a feature DpiApp provides on the viewer's side, when you enable PDF Downloading on your gallery setting, your client can then download a PDF containing all images of this gallery through the share link you provide them. If you want to download this PDF collection yourself, you can also click the share link you might want to give your client to go to the viewer side of this gallery, and download the PDF there.