I downgraded the device to earlier firmware (v.5.17.1736) from the
official support page, and it turned out that the PDF in question works fine. As soon as I upgraded the reader, the problem returned. I've had a look at all the Adobe and font folders in the two versions, but was unable to spot any difference – the files are of the same size and date in Total Commander.
In other words, the problem probably lies with the latest firmware, which makes the reader worse in this respect – a very disappointing experience given that the device is positioned as the company's premium ebook reader.
(I've recently discovered it has yet another problem: it cannot properly display pages with two and three columns; this, however, is endemic to Pocketbooks – I've tried an earlier model of Pocketbook Basic with the same result...)