It looks like Adobe withdrew the PDF with the file format
specifications for Photoshop 6.0.
In order to get the Photoshop 7.0 SDK, one needs to be a member of
the Adobe Solution Network. The cheapest variant will set you back
199 US$ or 229 euro, but even that does not guarantee access to the
Photoshop file format specs: Adobe will review every request for
access per case and gives itself the right to refuse to send you the
SDK, even if you are a paying member.
Google shows at least one mirror (in Japan) of the old specs. I don't
know if it is legal to download those. This may differ between
The 6.0 specs seem to indicate that the file format changes slowly,
so reverse engineering the formats should work in most cases.
Gimp-developer mailing list