After purchase, the download link appears. Click it to save the file as FKM_Richtlinie_7_Auflage.pdf (or similar name) to your computer.
The phrase in your search query suggests you have encountered a common confusion: the FKM PDF is often not a simple, double-clickable document. It is a protected PDF that requires an "installation" step. fkm richtlinie download file type pdf downloads install