Hi everyone
Revit models must be cloud workshared to be visible as linked files in Docs viewer.
Follow these steps. It worked for me
LINKED FILE (NON WORKSHARED MODEL)
OPEN THE LINKED FILE > COLLABORATE > COLLABORATE IN BIM 360 DOCUMENT MANAGEMENT > BROWSER PATH > INITIATE
MASTER FILE (NON WORKSHARED MODEL)
OPEN THE MASTER FILE > COLLABORATE > COLLABORATE IN BIM 360 DOCUMENT MANAGEMENT > BROWSER PATH > INITIATE
MANAGE LINK > ADD/RELOAD FROM > NAVIGATE TO EXTERNAL RESOURCES (CLOUD SYMBOL) > SELECT PROJECT > NAVIGATE TO FOLDER > SELECT LINKED FILE
COLLABORATE> MANAGE CLOUD MODELS > PUBBLISH LATEST
NOTE: IF FILES IN EXTERNAL RESOURCES ARE MISSING WHEN SELECTING LINKED FILE CHECK THIS OUT
https://knowledge.autodesk.com/support/revit/troubleshooting/caas/sfdcarticles/sfdcarticles/Empty-Op...