Al Rosen said:
You need to keep a copy of the document for each revision.
Looking to the international standard is clear that only obsolete quality documents with legal or preservation purpose could be retained but is not so clear for how long time. (see 9001:2000 4.2.3g any obsolete documents retained for legal and/or Knowledge preservation purposes are suitably identified. or 17025:2000 4.3.2.2d obsolete documents retained for either legal or knowledge preservation purposes are suitably marked).
To comply with the clause, you should mark all retained documents that are obsolete as such, either with a stamp, written on the document, or in the case of electronically preserved documents, by moving them to a file that indicates that they are obsolete. E- (from laboratory accrediataion bureau - Guidance for Documenting and Implementing ISO/IEC 17025).
For what it regards my experience, after archiving an obsolete document, there was only few occassion where I went again to read it, and auditors too.
External documents could be an exception.
The retention time of any old copies of customers specifications, I think should be more longer than others.
Anyway even if old revisions are often useless, remember that the third part auditors are always from the part of the reason.......... so i suggest you to collect all the old revision of the documents (possible in pdf format).By