Error: Type PdfViewerComponent is part of the declarations of 2 modules

thank you both for your responds. You were absolutely right.
the solution is:
in app.module.ts

import { PdfViewerModule } from ‘ng2-pdf-viewer’;
imports: [
BrowserModule,
…
PdfViewerModule
],

in page.module.ts

import { PdfViewerModule } from ‘ng2-pdf-viewer’;
imports: [
IonicPageModule.forChild(Page),
PdfViewerModule
],