Not a DOI
The resolution service at https://doi.org resolves DOIs. You have attempted to resolve content/pdf/bfm:978-1-4020-4246-1/5/1, which is not a DOI.
content/pdf/bfm:978-1-4020-4246-1/5/1 may be a Handle (DOIs are a special sort of Handle). If content/pdf/bfm:978-1-4020-4246-1/5/1 is a valid Handle you should be able to resolve it at https://hdl.handle.net.
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