Put another way... If you're creating the documents yourself, rather than getting them as PDF from another source, why are you creating PDFs that you need to zoom in to be able to read? :-)
The second part was guessing that you were using the default settings of the creation/conversion software, but the chances are that it would be expecting to be producing a document that would be printed, so would expect the document to be output onto a device(printer) with an A4 or Letter page size. However, by trying to read it's output on the Sony, it would be actually displaying it on a device with a page size of less than a 1/4 of the 'intended' size, meaning that you had to zoom it to see it!
While the easiest option is probably to convert to LRF or RTF, the other option would be to see if you could produce anything usable when targeting a page size of ~8cm x 11cm.
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