Unfortunately, I couldn't find a free and legitimate source for downloading the ISO 11137-3 PDF. The standard is copyrighted by ISO, and it's not publicly available for free download. You can, however, purchase the standard from the ISO website or other authorized distributors.
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If you're interested in accessing the standard, I recommend purchasing it from a reputable source to ensure you're getting an authentic and up-to-date copy. You can also consider contacting your local national standards body or a library that provides access to standards for assistance.