Choose SPO,
@microsoft dependencies should be ~1.4.1 in package.json file
"dependencies": { "@microsoft/sp-core-library": "~1.4.1", "@microsoft/sp-webpart-base": "~1.4.1", "@microsoft/sp-lodash-subset": "~1.4.1", "@microsoft/sp-office-ui-fabric-core": "~1.4.1", "@types/webpack-env": ">=1.12.1 <1.14.0" }, "devDependencies": { "@microsoft/sp-build-web": "~1.4.1", "@microsoft/sp-module-interfaces": "~1.4.1", "@microsoft/sp-webpart-workbench": "~1.4.1" }
$wepApp = Get-SPWebApplication("https://site")
$webApp.AllowedInlineDownloadedMimeTypes.Add("application/pdf")
$webApp.Update()
or use Central Admin Panel