is it possible that visitors of my site can choose images they like (maybe by a small icon on every image or a "add to pdf button") to create a downloadable pdf?
image-pdf for download
However this would require a lot of customisation and you would need good knowledge of Html, CSS & JS.
Lay Theme is built in a way that you have the ability to add Custom Code, with this feature a lot is possible.
I apologise that i cannot go through the process on this thread, but is do wish you the very best as this is a wonderful idea :)
If this is out of your abilities, seeking the help of a developer may be of interest to you!
Best wishes and Good luck! :)
Thank you for using Lay Theme and have a wonderful day
If you want one pdf containing all selected images you need custom coding or maybe there is a plugin; if it's ok to have a pdf for every image it'd be quite easy by allowing the filetype and then linking a thumbnail to the file.