Knit to PDF only opens preview the first time

I have your configuration but didn't experience the same issue. Here's what I did:

  1. Created a new RMarkdown file with the pdf option
  2. Knit
  3. Saw preview
  4. Changed the chuck with plot(pressure) to plot(cars)
  5. Knit
  6. Saw preview

Can you try that to eliminate Mojave/RStudio minimal issues and then post of reproducible example, called a reprex of an Rmd file that's giving your the problem?