The tmap_save() function guesses the expected graphic devices from the file extension. This is great in most cases, however, it does not allow to use any other graphic devices except the few that are build-in. This is in contrast to the ggsave(), which has the device argument - https://ggplot2.tidyverse.org/reference/ggsave.html.

Why does anyone want to use any other graphic devices? (1) they could render things nicer than some default graphic devices, (2) they could allow using foreign characters or external fonts, (3) ragg is not the default ggplot2 graphic device for raster graphical formats, however, it cannot be used with tmap_save(). See a small motivational example below.

@mtennekes, would it be possible to add the device argument to tmap_save()?

library(tmap)
#> Registered S3 methods overwritten by 'stars':
#>   method            from
#>   st_crs.SpatRaster sf  
#>   st_crs.SpatVector sf
library(spData)

# adds foreign characters
nz$Name2 = paste0(nz$Name, "śżć")

# creates a map
tm = tm_shape(nz) +
  tm_text(text = "Name2", fontfamily = "sans", fontface = "italic")
tm

# saves using tmap_save
tmap_save(tm, "tm.pdf")
#> Warning in grid.Call(C_textBounds, as.graphicsAnnot(x$label), x$x, x$y, :
#> conversion failure on 'Northlandśżć' in 'mbcsToSbcs': dot substituted for <c5>
#> Map saved to /tmp/RtmpWIcJli/reprex-65256290a1b4-wax-mouse/tm.pdf
#> Size: 5.819444 by 8.402778 inches

tm.pdf

# saves using external device
cairo_pdf("tm2.pdf")
tm
dev.off()
#> png 
#>   2

tm2.pdf

^{Created on 2021-08-11 by the reprex package (v2.0.1)}