Struggling with PDF output of bookdown - pdf

Question

I thought it would be a good idea to write a longer report/protocol using bookdown since it's more comfortable to have one file per topic to write in instead of just one RMarkdown document with everything. Now I'm faced with the problem of sharing this document - the HTML looks best (except for wide tables being cut off) but is difficult to send via e-mail to a supervisor for example. I also can't expect anyone to be able to open the ePub format on their computer, so PDF would be the easiest choice. Now my problems:
My chapter headings are pretty long, which doesn't matter in HTML but they don't fit the page headers in the PDF document. In LaTeX I could define a short title for that, can I do that in bookdown as well?
I include figure files using knitr::include_graphics() inside of code chunks, so I generate the caption via the chunk options. For some figures, I can't avoid having an underscore in the caption, but that does not work out in LaTeX. Is there a way to escape the underscore that actually works (preferrably for HTML and PDF at the same time)? My LaTeX output looks like this after rendering:
\textbackslash{}begin\{figure\}
\includegraphics[width=0.6\linewidth,height=0.6\textheight]{figures/0165_HMMER} \textbackslash{}caption\{Output of HMMER for PA\_0165\}\label{fig:0165}
\textbackslash{}end\{figure\}
Edit
MWE showing that the problem is an underscore in combination with out.height (or width) in percent:
---
title: "MWE FigCap"
author: "LilithElina"
date: "19 Februar 2020"
output: pdf_document
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```
## R Markdown
This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.
When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:
```{r cars}
summary(cars)
```
## Including Plots
You can also embed plots, for example:
```{r pressure, echo=FALSE, fig.cap="This is a nice figure caption", out.height='40%'}
plot(pressure)
```
Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.
```{r pressure2, echo=FALSE, fig.cap="This is a not nice figure_caption", out.height='40%'}
plot(pressure)
```

Answer 1

Concerning shorter headings: pandoc, which is used for the markdown to LaTeX conversion, does not offer a "shorter heading". You can do that yourself, though:
# Really long chaper heading
\markboth{\thechapter~short heading}{}
[...]
## Really long section heading
\markright{\thesection~short heading}
This assumes a document class with chapters and sections.
Concerning the underscore in the figure caption: For me it works for both PDF and HTML to escape the underscore:
```{r pressure2, echo=FALSE, fig.cap="This is a not nice figure\\_caption", out.height='40%'}
plot(pressure)
```