How to navigate back from file to slideshow with no keyboard/ mouse

We have a large touch screen at our event and will run a slideshow from this for visitors to browse a series of articles (saved as pdfs). I can link the pdf to a slide and when running in slideshow mode, the pdf opens perfectly, but how do I force the user back to the slideshow once they've finished reading the pdf? There is no keyboard or mouse of anything attached to the touch screen.

Any ideas on a solution even if it's a workaround? Thank you so much.

(Note we have 50 slides, each one links to a pdf document. Some pdf documents are 130 pages long)