ffprobe video.mkv 2>&1 | grep Subtitle

This will generate something like this:

Stream #0:3(eng): Subtitle: subrip (srt) (default) (forced)
Stream #0:4(eng): Subtitle: subrip (srt)
Stream #0:5(eng): Subtitle: subrip (srt) (hearing impaired)
Stream #0:6(dut): Subtitle: subrip (pgs)

We're looking for a Dutch (dut) PGS subtitle, so that would be stream #0:6 in this example.

Now we can extract this PGS subtitle as a SUP file:

ffmpeg -i filename.mkv -map 0:s:6 -c:s copy -f sup output.sup

(here "0:s:6" stands for stream #0:6 - the pgs subtitle we want)

No you can use Subtitle Edit (only available for Windows, so you will need a virtual machine or a Windows PC) to convert this with OCR to a plain text SRT.