Pause next slide until all documents are opened.

I have 5 buttons on a slide,  the first four will allow different PDF files to open when each button is pressed.  The 5th button goes to next slide when pressed.  The 5th button is the only one that has a delay.  Currently, when the learner presses one of the buttons to open a pdf, it does what is supposed to happen.  In a new tab, the pdf opens, but when I come back to the tab with the lesson, the slide goes to the next slide.  This happens without the 5th button being pressed.  How do I prevent this from happening unless the learner presses the 5th button.

Thank you.