Попробуйте добавить ссылку типа
<a [href]="downloadLink">Download {{ myfile.filename }}</a>
А в вашем классе,
//First import
import { DomSanitizer } from '@angular/platform-browser';
// Then
constructor(private sanitizer: DomSanitizer) {}
myfile = {
filetype: "octet-stream;base64",
filename: "test.pdf",
base64: 'JVBERi0xLjUNCiW1tbW1DQoxIDAgb2JqDQo8PC9UeXBlL0NhdGFsb2cvUGFnZXMgMiAwIFIvTGFuZyhlbi1HQikgL1N0cnVjdFRyZWVSb290IDEwIDAgUi9NYXJrSW5mbzw8L01hcmtlZCB0cnVlPj4'
};
downloadLink = this.sanitizer.bypassSecurityTrustUrl(`data:${this.myfile.filetype},${this.myfile.base64}`);