diff --git a/src/mixins/AttachmentMixin.js b/src/mixins/AttachmentMixin.js
index f214b4ef4b..5ecd559c66 100644
--- a/src/mixins/AttachmentMixin.js
+++ b/src/mixins/AttachmentMixin.js
@@ -3,6 +3,20 @@
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
 
+const PDF_MIME = 'application/pdf'
+const PDF_HANDLER_ID = 'pdf'
+
+/**
+ * Mail attachments have no Nextcloud file id, so office handlers that hijack
+ * application/pdf (Nextcloud Office, ONLYOFFICE) cannot open them and bail out
+ * with an empty WOPI config. Only files_pdfviewer works off a plain URL.
+ *
+ * @return {boolean} whether files_pdfviewer is registered in the viewer
+ */
+function hasPdfViewer() {
+	return (OCA?.Viewer?.availableHandlers ?? []).some((handler) => handler.id === PDF_HANDLER_ID)
+}
+
 export default {
 	computed: {
 		fileInfos() {
@@ -24,7 +38,7 @@ export default {
 			return this.fileInfos.filter((fileInfo) => (fileInfo.mime.startsWith('image/')
 				|| fileInfo.mime.startsWith('video/')
 				|| fileInfo.mime.startsWith('audio/')
-				|| fileInfo.mime === 'application/pdf')
+				|| (fileInfo.mime === PDF_MIME && hasPdfViewer()))
 			&& OCA.Viewer.mimetypes.includes(fileInfo.mime))
 		},
 	},
@@ -37,10 +51,17 @@ export default {
 				return
 			}
 
-			OCA.Viewer.open({
+			const options = {
 				fileInfo,
 				list: this.previewableFileInfos,
-			})
+			}
+
+			if (fileInfo.mime === PDF_MIME && OCA.Viewer.openWith) {
+				OCA.Viewer.openWith(PDF_HANDLER_ID, options)
+				return
+			}
+
+			OCA.Viewer.open(options)
 		},
 
 	},
diff --git a/src/tests/unit/mixins/AttachmentMixin.spec.js b/src/tests/unit/mixins/AttachmentMixin.spec.js
index 45a71607e4..eb701428b7 100644
--- a/src/tests/unit/mixins/AttachmentMixin.spec.js
+++ b/src/tests/unit/mixins/AttachmentMixin.spec.js
@@ -9,6 +9,7 @@ import AttachmentMixin from '../../../mixins/AttachmentMixin.js'
 const attachments = [
 	{ downloadUrl: '/cat.png', fileName: 'cat.png', mime: 'image/png' },
 	{ downloadUrl: '/notes.txt', fileName: 'notes.txt', mime: 'text/plain' },
+	{ downloadUrl: '/invoice.pdf', fileName: 'invoice.pdf', mime: 'application/pdf' },
 ]
 
 function mountMixin() {
@@ -25,7 +26,7 @@ describe('AttachmentMixin', () => {
 	})
 
 	it('previews attachments supported by the viewer', () => {
-		global.OCA = { Viewer: { mimetypes: ['image/png'] } }
+		global.OCA = { Viewer: { mimetypes: ['image/png'], availableHandlers: [] } }
 
 		const view = mountMixin()
 
@@ -42,4 +43,44 @@ describe('AttachmentMixin', () => {
 		expect(view.vm.previewableFileInfos).toEqual([])
 		expect(view.vm.canPreview(view.vm.fileInfos[0])).toBe(false)
 	})
+
+	it('does not preview pdf attachments without files_pdfviewer', () => {
+		global.OCA = {
+			Viewer: {
+				mimetypes: ['application/pdf'],
+				availableHandlers: [{ id: 'onlyoffice', mimes: ['application/pdf'] }],
+			},
+		}
+
+		const view = mountMixin()
+
+		expect(view.vm.previewableFileInfos).toEqual([])
+		expect(view.vm.canPreview(view.vm.fileInfos[2])).toBe(false)
+	})
+
+	it('opens pdf attachments with files_pdfviewer', () => {
+		const openWith = vi.fn()
+		const open = vi.fn()
+		global.OCA = {
+			Viewer: {
+				mimetypes: ['application/pdf', 'image/png'],
+				availableHandlers: [{ id: 'onlyoffice' }, { id: 'pdf' }],
+				openWith,
+				open,
+			},
+		}
+
+		const view = mountMixin()
+		view.vm.showViewer(view.vm.fileInfos[2])
+
+		expect(openWith).toHaveBeenCalledWith('pdf', {
+			fileInfo: view.vm.fileInfos[2],
+			list: view.vm.previewableFileInfos,
+		})
+		expect(open).not.toHaveBeenCalled()
+
+		view.vm.showViewer(view.vm.fileInfos[0])
+
+		expect(open).toHaveBeenCalledTimes(1)
+	})
 })