您可能需要使用LaTeX的caption包和justification=centering选项（即YAML部分的header-includes字段中的\usepackage[justification=centering]{caption}）。

---
header-includes:
  - \usepackage[justification=centering]{caption}
output:
  pdf_document: default
---

```{r setup, echo=FALSE, warning=FALSE, message=FALSE}
library(tidyverse)
library(kableExtra)

kbl(mtcars,
  caption = "This is a really long title that will go past the borders when knitting to PDF in portrait mode. I'd love to figure out how to keep it centered even if the text is really long.",
  booktabs = TRUE, linesep = "", align = "c"
)  %>%
  kable_styling(latex_options = c("striped", "hold_position"))

![](https://i.stack.imgur.com/jNzF8.png)

如果没有@卡洛斯Luis里维拉展示的额外LaTeX包，我无法找到任何可能的方法通过使用kable来居中标题。因此，如果您不需要额外的LaTeX包，我建议使用flextable包而不是kable和kableExtra。在下面的示例中，我使用pdf-latex：

```{r}
library(flextable)
library(tidyverse)

set_flextable_defaults(fonts_ignore = TRUE)
mtcars |> rownames_to_column(var = "carmodel") |> 
    flextable() |> theme_zebra() |>
    align(j = ~.-carmodel, align = "center", part = "all") |>
    autofit(add_w = 0) |>
    set_caption(caption = "This is a really long title that will go past the borders 
        when knitting to PDF in portrait mode. I'd love to figure out how to keep it 
        centered even if the text is really long.")

生成的PDF表格：