The Tool Desk
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The three controls you must keep separate
A reliable device-sized PDF starts by separating three stages of Chromium’s pipeline:
- Viewport emulation:
widthandheightare CSS pixels.deviceScaleFactordescribes the emulated display density and does not choose PDF paper size. - CSS media:
page.pdf()uses theprintmedia type by default. If the PDF should look like the screen, callpage.emulateMediaType('screen')before creating it. - PDF page and rendering:
format,width,height, CSS@page,preferCSSPageSize, and PDFscalecontrol the output document.
Changing only the viewport will not make a PDF page physically match that viewport. Conversely, increasing PDF scale will not emulate a higher-density phone display.
A complete Puppeteer implementation
This example emulates a 390 × 844 CSS-pixel mobile device, keeps screen styles, and creates a PDF page with the same nominal CSS dimensions.
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const puppeteer = require('puppeteer');
(async () => {
const browser = await puppeteer.launch({headless: true});
const page = await browser.newPage();
// Device metrics are CSS pixels; DPR is a separate property.
await page.setViewport({
width: 390,
height: 844,
deviceScaleFactor: 3,
isMobile: true,
hasTouch: true
});
await page.goto('https://example.com', {
waitUntil: 'networkidle2',
timeout: 60000
});
// PDF defaults to print CSS. Select screen CSS explicitly when required.
await page.emulateMediaType('screen');
await page.pdf({
path: 'device-viewport.pdf',
width: '390px',
height: '844px',
preferCSSPageSize: true,
scale: 1,
printBackground: true,
margin: {top: '0px', right: '0px', bottom: '0px', left: '0px'}
});
await browser.close();
})();Replace the URL with your page and adjust the dimensions to the target profile. The values above illustrate how the settings relate; they are not a universal conversion for every stylesheet or desired paper format.
Set the intended device before navigation
Some sites choose layout, scripts, or resources during the first navigation. Configure the viewport before goto() so the page sees the intended metrics from the start. page.emulate(device) is a shortcut that applies a device’s user agent and viewport and resizes the page; use it before navigation when using a built-in device descriptor.
const devices = require('puppeteer').KnownDevices;
await page.emulate(devices['iPhone 13']);
await page.goto(url, {waitUntil: 'networkidle2'});If you need a custom profile, use setViewport(). Remember that mobile layout also depends on the document’s viewport meta tag. A page without an appropriate <meta name="viewport"> can render with a layout viewport that does not resemble the physical device you had in mind.
Choose screen or print CSS deliberately
Screen-style output
Call await page.emulateMediaType('screen') immediately before page.pdf(). This selects the screen media rules but does not set paper dimensions, margins, or PDF scale.
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Print-style output
Do nothing when print styling is desired: PDF generation uses print CSS by default. Check @media print rules for hidden navigation, changed colors, or altered widths before diagnosing a sizing problem as a viewport issue.
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Backgrounds and pagination
Set printBackground: true when colors and background images matter. For long pages, use normal pagination and inspect page breaks; a device-shaped page can still produce many PDF pages if its content exceeds the selected height.
Define PDF paper dimensions
Use a named format
format: 'A4', 'Letter', and other supported formats are convenient for documents intended for printers. When format is present, it takes priority over width and height.
Use explicit width and height
For a viewport-shaped page, provide CSS lengths such as width: '390px' and height: '844px'. You can use units such as in, cm, or mm when the output has a physical paper requirement.
Let CSS choose the page size
Declare the intended page in your stylesheet:
@page {
size: 390px 844px;
margin: 0;
}
html, body {
margin: 0;
}Then set preferCSSPageSize: true. Its documented default is false; with the default, Chromium scales content to fit the PDF paper size instead of giving CSS page size priority.
Understand PDF scale versus device scale factor
PDFOptions scale is a rendering adjustment with a documented range of 0.1 to 2 and a default of 1. It can make content fit or occupy more of a page, but it does not increase the emulated viewport’s CSS-pixel density. Keep deviceScaleFactor at the value needed to reproduce responsive or DPR-dependent behavior, and change PDF scale only after verifying media type, page dimensions, margins, and CSS page-size precedence.
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There is no universal formula that converts a phone’s DPR into a correct PDF scale. A page’s CSS, paper units, fonts, images, and Chromium version all affect the result. Generate representative files and inspect their actual dimensions and line wrapping.
Options that commonly affect apparent size
margin: Nonzero margins reduce the usable content area and can trigger fitting or extra pages.printBackground: Required when visual comparison depends on background paint.displayHeaderFooter: Headers and footers consume space and can change pagination.pageRanges: Useful for exporting selected pages while debugging a long document.landscape: Rotates standard paper formats; it does not rotate the emulated viewport.waitForFontsand application readiness: Wait for fonts, images, and client-side layout before capture so late reflow does not look like a scaling defect.
A repeatable sizing workflow
- Write down the target device’s CSS viewport width and height, DPR, mobile flag, and user agent.
- Apply those metrics (or
page.emulate()) before navigation. - Navigate with an explicit readiness condition such as
networkidle2, a selector, or an application-specific promise. - Decide whether the PDF should use print or screen CSS.
- Choose one paper strategy: a named format, explicit dimensions, or CSS
@pagewithpreferCSSPageSize: true. - Set margins and background printing intentionally.
- Start with PDF
scale: 1. Change it only when the page geometry is otherwise correct. - Compare PDFs for every supported profile, including narrow and wide breakpoints, long content, and pages with lazy-loaded images.
Troubleshooting mismatched PDFs
Everything is desktop-sized
Check that the viewport was set before goto(), that the mobile flag and user agent are correct, and that the page has a suitable viewport meta tag. A PDF paper setting cannot force responsive CSS to select a mobile breakpoint.
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Screen styles are missing
Call emulateMediaType('screen') before pdf(). If print rules still appear, confirm no later code switches the media type.
The page is unexpectedly shrunk
Look for format overriding width and height, nonzero margins, and preferCSSPageSize: false. Remove format when explicit dimensions should win, or enable CSS page-size preference.
Content is clipped or split
Inspect the selected page height, CSS @page rules, fixed-position elements, and overflow containers. A viewport height is not a guarantee that all document content fits on one page.
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Images or fonts change after capture
Wait for the relevant network requests or DOM state, preload critical fonts, and ensure lazy content has been triggered. Capture only after layout is stable.
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That is expected when the page does not use DPR-dependent assets or CSS. DPR affects emulation; PDF paper and PDF scale remain separate.
Performance, reliability, and cost considerations
Emulating a larger DPR can increase raster work and memory use, especially on image-heavy pages, but it does not replace selecting an appropriate PDF page size. Reuse a browser process for batches, close pages when finished, and apply navigation and capture timeouts. For reproducibility, pin the Puppeteer/Chromium version used in CI and keep viewport, media type, paper dimensions, margins, and scale in configuration rather than scattering them through scripts.
Validate at least one short page, one long page, a page with web fonts, and a page with responsive images. Store the chosen profile alongside the PDF so a later change in CSS or Chromium can be traced to a specific setting.
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Read the parameter reference in the ScreenshotNeo documentation. A cURL request is:
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpThe equivalent Python call:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
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Frequently Asked Questions
Does a higher deviceScaleFactor make a PDF sharper?
Not by itself. It changes emulated device metrics; PDF sharpness and geometry depend on Chromium’s PDF rendering, page dimensions, assets, and PDF options.
Should I use CSS @page or Puppeteer width and height?
Use CSS @page when the document owns its print geometry and set preferCSSPageSize to true. Use width and height when the capture script should enforce a profile independently of page CSS.
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