For reference, on the PRS600, overlined text is stored like that:
With a free-hand comment attached:
<annotation date="Fri, 23 Oct 2009 12:17:40 GMT" name="123456789ABCDEFGHIJKLMNOP Line 5" page="0" pageOffset="0" pages="1" part="0" scale="0" synced="true" dpi="167" width="584" height="754" layoutVersion="1"> <start>I3BkZmxvYyg1OTBlLDAsNDEsMCwwLDAsMCwxKQA=</start>
<end>I3BkZmxvYyg1OTBlLDAsNDEsMTgsMSwwLDEsMSkA</end>
<svgFile width="446" height="315">1256300260258.742.svg</svgFile>
<comment></comment>
</annotation>
With a text comment attached:
<annotation date="Fri, 23 Oct 2009 12:18:07 GMT" name="123456789ABCDEFGHIJKLMNOP Line 6" page="0" pageOffset="0" pages="1" part="0" scale="0" synced="true" dpi="167" width="584" height="754" layoutVersion="1"><start>I3BkZmxvYyg1OTBlLDAsNDksMCwwLDAsMCwxKQA=</start> <end>I3BkZmxvYyg1OTBlLDAsNDksMTgsMSwwLDEsMSkA</end>
<comment>jolly good</comment>
</annotation>
With none of the above:
<annotation date="Wed, 21 Oct 2009 14:24:19 GMT" name="23456789ABCDEFGHIJKLMNOP Line 7" page="0" pageOffset="0" pages="1" part="0" scale="0" synced="true" dpi="167" width="584" height="754" layoutVersion="1"><start>I3BkZmxvYyg1OTBlLDAsNTcsMCwwLDAsMSwxKQA=</start>
<end>I3BkZmxvYyg1OTBlLDAsNTcsMTgsMSwwLDEsMSkA</end>
<comment></comment>
</annotation>
The weird text between the start and end tags is just the "pdfloc" reference, in base64 encoding. I don't understand why someone would want to encrypt this kind of thing, and why with this kind of encryption.