# Conversion This page explains how `convert()` renders PDF pages into images, and what actually happens with resolution, output format, and failures. ## Flow 1. Sets `pdfjsLib.GlobalWorkerOptions.workerSrc` to pdf.worker.js on cdnjs 2. Parses `file` with `getDocument()`, fetching CMaps from cdnjs (for CJK and other font encodings) 3. Appends the progress overlay to `` once parsing succeeds 4. Creates a `canvas` for every page and renders them concurrently; each finished page pushes its Data URL into the result array and updates progress 5. Waits 500ms after the last page, resolves with the result array, and removes the overlay ## Resolution: `scale` Output pixel size = PDF page size (pt) × `scale`. The default is `1.5`; a US Letter page (612×792 pt) renders at 612×792 with `scale: 1` and at 918×1188 with the default. ```demo ``` ## Output Format: `type` `type` accepts only `png`, `jpg`, and `webp`; any other value falls back to `jpg`. Results come from `canvas.toDataURL("image/" + type)`, so: | `type` | Passed to `toDataURL` | Actual output | |---|---|---| | `png` | `image/png` | PNG | | `webp` | `image/webp` | WebP (PNG where the browser lacks WebP encoding) | | `jpg` (default) | `image/jpg` | **PNG**: browsers only recognize `image/jpeg` and fall back to PNG for unknown MIME types | In other words, the current version cannot produce JPEG, and the default actually yields PNG. ```demo ``` ## Result Order Pages render concurrently, and each result is pushed onto the array when its page finishes rather than written at its page index. When page complexity varies a lot, `images[0]` is not guaranteed to be page 1. ## `file` Is Single-Use pdf.js transfers the `ArrayBuffer` to its worker, so after conversion the original buffer's `byteLength` becomes 0. As a result: - A second `convert()` on the same instance rejects with a `TypeError` whose message is "Cannot perform Construct on a detached ArrayBuffer"; the already converted `images` stay intact - The same buffer cannot be handed to another instance; read the file again to reconvert ```demo ``` ## Failure Behavior | Case | Rejection value | |---|---| | No `file` provided | String `"error The PDF file is empty, i.e. its size is zero bytes."` | | Not a valid PDF | String `"error Invalid PDF structure."` | | A page fails to render | String `"error i is not defined"`: the error handler references a variable that does not exist, so the original message is lost | | Reusing a transferred buffer | A `TypeError` object | | pdf.js not loaded yet | A `ReferenceError` object (see [Core Concepts](/core-concepts)) | Rejection values mix strings and `Error` objects; `String(err)` handles both. For pdf.js load failures the library also calls `console.error("error", err)` itself. ```demo ``` ## Related Pages - [Progress Overlay](/progress-overlay) - [Methods and Properties](/api-reference-methods)