Lembar

A lightweight PDF reader for Android, built as an installable PWA with an optional Android app wrapper. Tap a PDF in WhatsApp and pick Lembar in "Open with" (Android app), or use Share → Lembar (PWA). Read offline and pick up where you left off.

Everything runs on the device. Files are never uploaded anywhere.

• Reading: continuous scroll, pinch-to-zoom, fit-to-width, page slider, jump to page, tap to hide the toolbars
• Zoom remembered per document, stored relative to fit-width so it carries over between phone, tablet and rotation
• Two-page spread on tablets and in landscape (automatic at 900px+ wide, or forced on/off with the view button). Page 1 stays alone like a book cover.
• Open from other apps: "Open with" for PDFs via the Android app (TWA), and the share sheet via the Web Share Target
• Recently read: thumbnails, progress, and auto-resume at the last page (keeps the 15 most recent files)
• Search: full-text search across all pages, with result snippets and in-page highlights
• Bookmarks per document, plus the PDF's own table of contents when it has one
• Night mode: inverts page colours for reading in the dark
• Offline: app shell and pdf.js are precached by the service worker
• Password-protected PDFs are supported
• On desktop Chrome/Edge, the installed app also registers as a .pdf file handler

sequenceDiagram
    participant WA as WhatsApp
    participant SW as sw.js
    participant App as index.html
    WA->>SW: POST /share-target (multipart, field "pdf")
    SW->>SW: store file in Cache Storage ("lembar-share")
    SW-->>App: 303 redirect to /?share=1
    App->>SW: read and delete the cached file
    App->>App: save to IndexedDB history, render with pdf.js

If the service worker is not active yet, src/worker.js catches the POST on Cloudflare and redirects to /?share=error instead of returning a 405.

• Vanilla HTML/CSS/JS in one index.html. No framework, no build step.
• pdf.js 3.11.174, vendored in public/vendor/ so it works offline
• IndexedDB for file history and bookmarks, localStorage for the night-mode preference
• Service worker: precached shell, stale-while-revalidate updates, Share Target receiver

.
├── public/                  # deploy this folder as-is
│   ├── index.html           # the whole app
│   ├── sw.js                # service worker (offline + share target)
│   ├── manifest.webmanifest # install metadata, share_target, file_handlers
│   ├── _headers             # Cloudflare cache/content-type headers
│   ├── sample.pdf           # built-in guide document
│   ├── icons/               # any + maskable icons (192, 512)
│   └── vendor/              # pdf.min.js, pdf.worker.min.js
├── src/
│   └── worker.js            # Worker: serves public/, assetlinks.json, share fallback
├── android/
│   └── twa-manifest.json    # Bubblewrap config for the Android app
├── .github/workflows/
│   └── android.yml          # builds and signs the APK
├── deploy/
│   └── Caddyfile            # alternative: self-host behind Caddy
└── wrangler.toml            # Workers + static assets config

python3 -m http.server 8080 -d public
# or: npx serve public

Open http://localhost:8080. Service workers are allowed on localhost. Anywhere else they need HTTPS.

The app is deployed as a Worker with static assets. wrangler.toml serves public/, and public/_headers sets the cache and content-type headers.

Option A: Git integration (recommended)

1. Cloudflare dashboard → Workers & Pages → Create → Import a repository → pick this repo.
2. Build settings:
   • Build command: (leave empty)
   • Deploy command: npx wrangler deploy
3. Make sure name in wrangler.toml matches the Worker's name in the dashboard (currently pdf-reader).
4. Every push to main redeploys automatically.
5. Optional: Settings → Domains & Routes → add a custom domain such as lembar.erwarx.com.

Option B: from your machine

npx wrangler deploy

Test locally with the same runtime:

npx wrangler dev

Copy public/ to the server and use deploy/Caddyfile. Caddy handles HTTPS automatically.

1. Open the deployed URL in Chrome for Android.
2. Tap Install on the page (or ⋮ → Install app). It has to be a real install, not "Add to Home screen", because only installed WebAPKs appear in the share sheet.
3. In WhatsApp: long-press a PDF → Share → Lembar.

A PWA can't appear in Android's "Open with" list, so the repo also builds a small Android app. It's a WebView shell: a full-screen WebView that loads this same site, with reading history, bookmarks and the offline cache stored inside the app. It registers as a handler for application/pdf ("Open with") and as a share target.

The project is generated with Bubblewrap (icons, signing, Gradle setup), then android/patch_project.py replaces its Trusted Web Activity launcher with android/LauncherActivity.java. A TWA was tried first, but Chrome dropped PDFs handed over from WhatsApp on some phones (Galaxy A05s), both through launchQueue and the share target.

sequenceDiagram
    participant WA as WhatsApp
    participant A as LauncherActivity
    participant W as WebView (index.html)
    WA->>A: ACTION_VIEW / ACTION_SEND content://… (application/pdf)
    A->>A: copy to cache/shared_pdfs/
    A->>W: load https://HOST/ (or evaluateJavascript if already open)
    W->>A: LembarNative.pending() → {id, name, size}
    loop chunks of ≤512 KB
        W->>A: LembarNative.read(id, offset, length) → base64
    end
    W->>A: LembarNative.done(id)
    W->>W: open with pdf.js

The app opens https://lembar.erwarx.com (set in the workflow; a TWA_HOST repository variable overrides it). The WebView shell doesn't need Digital Asset Links; the Worker still serves /.well-known/assetlinks.json from wrangler.toml in case you go back to a TWA.

The only thing GitHub needs is the signing key, as two repository secrets (Settings → Secrets and variables → Actions → Secrets):

• ANDROID_KEYSTORE_BASE64: the keystore file, base64-encoded (base64 -i lembar.keystore on macOS, base64 -w0 lembar.keystore on Linux)
• ANDROID_KEYSTORE_PASSWORD: the keystore password

Keep a backup of the keystore and password somewhere safe. Every future update must be signed with the same key, or Android will refuse to install it over the old version.

Create a new one, update both secrets, and replace TWA_SHA256 in wrangler.toml (both [vars] and [previews.vars]) with the new fingerprint. Phones with the old build have to uninstall it first.

keytool -genkeypair -keystore lembar.keystore -storetype PKCS12 -alias lembar \
  -keyalg RSA -keysize 2048 -validity 10000 -dname "CN=Lembar"
keytool -list -v -keystore lembar.keystore -alias lembar | grep SHA256

Push a tag such as v1.0.0. The workflow builds a signed APK and attaches it to a GitHub Release. Download it on the phone, open it, and allow installing from that source when Android asks.

Pull requests that touch android/ build a test APK signed with a throwaway key. It's useful to check that the build works, but not for installing.

• Android System WebView up to date (updated through the Play Store)
• Android 7.0+ (minSdkVersion 24, required by androidbrowserhelper)

Bump VERSION in public/sw.js (e.g. lembar-v4) on every deploy that changes cached files. This makes the old cache get dropped. sw.js, / and the manifest are served with Cache-Control: no-cache, so clients pick up the new worker on their next visit.

• "Open with" needs the Android app. Chrome on Android doesn't support file_handlers for installed PWAs, so only the Android app appears there. The plain PWA gets the share sheet.
• Separate storage. The Android app keeps its own history and bookmarks, separate from the PWA in Chrome.
• iOS supports neither "Open with" nor share targets for web apps. Use the in-app Buka PDF button.
• Scanned PDFs are images with no text layer, so search finds nothing in them.
• Storage is per browser. History and bookmarks live in that browser's IndexedDB and are lost if site data is cleared.
• Fonts load from Google Fonts and are cached after the first visit. On a cold offline start, system fonts are used.

• TWA wrapper (Bubblewrap) + /.well-known/assetlinks.json for "Open with"
• Remember zoom level per document
• Two-page spread on tablets / landscape
• Publish the Android app on Google Play