Comment Standards!!! (Score 1) 633
For the data format: stick to documented standards. ASCII or UTF-8 Unicode will do great for the text of a document; (X)HTML is likely to be available too; PDF maybe. For pictures I'ld bet on JPEG or an uncompressed RGB format, for moving images on MPEG2. There is nothing wrong with storing files in multiple formats for redundancy.
The medium is another issue. Would a CD-R be readable after 15 years? A CD-RW may be more reliable, but can you find a CD-ROM player at that time? A USB stick or SD card are "new" media where readers are likely to be available, but little is known about long-term data persistence. Having a backup of the data on an actively backed up computer does not sound like a bad idea at all.
Summary: with more baskets, the likelihood of remaining eggs increases.