The free to use online PDF library at BuildInstructions.com
has a section dedicated to collecting Old Warhammer Model Catalogues and Old Warhammer Annuals.
These brilliant publications have all been captured digitally and converted to PDF's which are free to download direct from the buildinstructions.com website.
There's some really awesome nostalgia and fantastic Old Hammer artwork to be re-discovered!
The Model Catalogues go as far back as 1980, and include colour copies of the original Citadel Miniatures Catalogues from the period, all with original artwork in high definition.
Interested in the old 1990's and early 2000's modelling and painting guides? - there's a handful of those too! There's an old Forge World Guide to Building Resin Models, and even a publication from an earlier White Dwarf magazine on How to Build a Baneblade from Plasticard (White Dwarf Modelling Workshop).
You can find all the currently uploaded PDF's of historic Games Workshop and Citadel Model Catalogues and Guides here.
Enjoy!