All models prior to the 900 (yes, 900, not 950) had a flaw wherein the device would not permit both a DRM'ed PDF and a DRM'ed Epub to be open at the same time on the same device. Non-restricted books were fine - a DRM in one format, and a non-DRM in the other was fine, just not both types, both DRM'ed. Sony resolved that for the 900, but (surprise! surprise!) couldn't have cared less about a retroactive fix for the older models.
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