RStudio Connect Shiny downloadHandler works with HTML but not PDF

RStudio Connect makes sure that the R packages needed to run your content are available but does not make similar accommodations for TeX/LaTeX packages.

You will need to work with your systems administrator to configure LaTeX on the system running RStudio Connect. The floatrow package is usually contained in a distribution-specific package. Ubuntu has the texlive-latex-extra package. I don't have the Red Hat package handy.

Here is a related Community post discussing installing LaTeX on CentOS for use with RStudio Connect: Texlive distribution on centOS for RStudio Server and Connect