Hi Bube,
Yes this is a know bug. At least I've experienced this defect and others here have mentioned it.
There is no ideal workaournd for this, but there are rumors of a Sony PDF fix.
For me the only PDF I read are those generated by myself or by sites like FeedBook that create custom PDF that are optimized for the reader.
For PDF of size A4/letter then a tool such as
PDFLRF can be used to make the text readable on the reader.
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