How to change the encoding in pdf documents?
I don't think it's the encoding it seems that none of the included fonts in dompdf support it. See also https://stackoverflow.com/questions/16384517/dompdf-character-encoding-utf-8. If you look in the vendor/dompdf/dompdf/lib/fonts folder there's a readme about DejaVu, and it says you can get the font from http://code.google.com/p/dompdf and copy it into your dompdf/lib/fonts folder. Then as the first link describes you need to change the font used by the Civi message template you're using, e.g. in the template under Administer - CiviMail - Message Templates if you edit the template you'll see either &lt;font&gt; tags or something like style="font-family: Arial", and you can change the font there. If you're pdf'ing reports you can edit css/print.css.

Note that clicking the .ttf files at the link above seem to be redirects to another page where you can actually download the real file. If you just save the .ttf files directly they aren't really ttf files.

Also in order to get it to recognize the .ttf I had to add this to the print.css file:

@font-face {
  font-family: 'DejaVu Sans';
  font-style: normal;
  font-weight: normal;
  src: url(https://mysite.org/path/to/civicrm/vendor/dompdf/dompdf/lib/fonts/DejaVuSans.ttf) format('truetype');
}
@font-face {
  font-family: 'DejaVu Sans';
  font-style: italic;
  font-weight: normal;
  src: url(https://mysite.org/path/to/civicrm/vendor/dompdf/dompdf/lib/fonts/DejaVuSans-Oblique.ttf) format('truetype');
}
@font-face {
  font-family: 'DejaVu Sans';
  font-style: normal;
  font-weight: bold;
  src: url(https://mysite.org/path/to/civicrm/vendor/dompdf/dompdf/lib/fonts/DejaVuSans-Bold.ttf) format('truetype');
}
@font-face {
  font-family: 'DejaVu Sans';
  font-style: italic;
  font-weight: bold;
  src: url(https://mysite.org/path/to/civicrm/vendor/dompdf/dompdf/lib/fonts/DejaVuSans-BoldOblique.ttf) format('truetype');
}