To include the Web.config file in the dest folder while running a build in Angular, you can modify the angular.json file as follows:

Angular.Json
Note: We need to include Web.config file inside the src folder.)

In build declaration, we can define the web config inside the block of assets.
"build": {
       "builder": "@angular-devkit/build-angular:browser",
       "options": {
         "outputPath": "dist",
         "index": "src/index.html",
         "main": "src/main.ts",
         "polyfills": "src/polyfills.ts",
         "tsConfig": "tsconfig.app.json",
         "inlineStyleLanguage": "scss",
         "assets": [
           "src/favicon.ico",
           "src/assets",
           "src/web.config"
         ],
         "styles": [
           "src/styles.scss"
         ],
         "scripts": [
           "node_modules/jquery/dist/jquery.min.js"
         ]
       }

     },

Another Way
In build declaration inside the asset block we can define the we.config with input and output.
{
"assests":{
"src/web.config",
              {
                "glob": "**/*",
                "input": "node_modules/ng2-pdfjs-viewer/pdfjs",
                "output": "assets/pdfjs"
              },
...
...
...
}
}