When I need to download a copy of a patent for myself or a client, I usually go right to esp@cenet. Since esp@cenet includes ~60 million patent documents from more than 70 countries, the odds of finding a patent or at least a family member are very good. However, sometimes I discover that esp@cenet won't let me download the patent I want because it is too long (~80 pages or more). This is no big deal if it's a US patent... there are plenty of other websites where I can download PDF copies of US patents. But if it's a patent from another country, things get a bit more complicated... Below are a few alternate patent sites that don't have page-limit restrictions.
PatentScope (WIPO) PatentScope is a database of published international PCT applications maintained by the World Intellectual Property Office. You can download copies of PCT applications and cited national priority applications in PDF or TIFF formats. If your patent or application was cited as priority document in a PCT application, you should be able to obtain a PDF copy from the PatentScope record. How do you know if your patent was cited in a PCT application? Easy: simply look up the patent family in esp@cenet and check any PCT applications that are included.
DEPATISnet (German PTO) DEPATISnet is esp@cenet's little brother, but what it lacks in size it makes up for in smarts. DEPATISnet doesn't cover nearly as many countries as esp@cenet but it does cover the major and several minor patent offices. Documents are stored in PDF and can be easily downloaded with no page limit restrictions. (I recently used DepatisNet to retrieve a copy of a 121-page German patent application from 1975.)
Patent Application Information Retrieval (PAIR) System (USPTO) If the desired foreign patent was cited in a US patent issued after June 30, 2003, a PDF copy should be available in the electronic file wrapper stored in the Public PAIR system. You can find US patents that cite foreign by searching the number in the FREF field in the PatFT database. If you get a hit, simply retrieve the US patent's file wrapper from PAIR and look up the foreign reference. All cited foreign patents are labeled "foreign reference", so you may have to look at a few until you find the right one.
I'm the librarian for research services in the Engineering and Science Library at Queen's University in Kingston, Ontario. I've been working with patent information since 1991, including seven years at the U.S. Patent and Trademark Office. I believe that the dissemination of patent information is a public good and should be promoted, especially in the education of science and engineering students.