Combining A5 images into a single A4 PDF

Hint and tip:

```
pdfjam --a4paper *.jpg --nup 2x1 --landscape --frame true --outfile pdfjam.pdf
```

https://askubuntu.com/questions/246647/convert-a-directory-of-jpeg-files-to-a-single-pdf-document#comment2705312_1190472