PDF to Image at 2x: Effective DPI and Measured File Sizes

When you convert a PDF to images, the resolution setting often says nothing more than "1x, 2x, 3x". Not knowing what that means in DPI, people reach for the highest option — and turn a 1.8 KB PDF into 209.5 KB of images across three pages, roughly 119 times bigger. We rendered three PDFs with different content through exactly the same path as PDF to Image (a browser canvas) and measured the real byte sizes at each scale and format. The short version: the right scale depends on what you will do with the images, and whether PNG or JPG is smaller flips depending on what is on the page.

The scale converts to effective DPI

PDF coordinates are in points, and 1 pt is exactly 1/72 inch. A scale of 1 means "draw one point as one pixel", so the effective rendering resolution is 72 DPI at 1x. Everything else is multiplication: 2x is an effective 144 DPI, and 3x is an effective 216 DPI. Checking against A4 (210 × 297 mm = 595 × 842 pt): 595 px ÷ 8.268 in = 72.0 dpi.

This is separate from any DPI recorded in the saved image. The HTML Standard says that where the file format can encode resolution metadata, it must be written as 96dpi — but what a browser actually writes depends on its canvas.toBlob() implementation. In the Chromium-based browser we measured, the PNG carried no resolution chunk (pHYs) and the JPG's JFIF header had no units (pixel aspect ratio 1:1). So while 2x does draw 144 pixels for each inch of the PDF page, the exported image does not necessarily record 144 as its DPI. If print software sizes an image solely from that metadata, use the pixel dimensions or set the intended DPI after export.

To work backwards from a physical print size instead, DPI & Print Size Calculator takes any two of pixels, DPI and output size and gives you the third.

How we measured

We prepared three single-page A4 PDFs that differ only in content and exported them the same way: pdf.js for rendering, canvas.toBlob() for export, JPG quality 0.92 (the same setting the tool uses), and a white background painted before drawing for JPG. Measurements used a Chromium-based browser.

• A — text with plenty of white space: the sample PDF bundled with this site (a 20pt heading plus a few lines of 12pt body text). The file itself is 1.8 KB.
• B — dense text: the same A4 page filled with 60 lines of 10pt body text.
• C — one photo: a single photograph covering the whole A4 page.

Measured: the same scale, opposite conclusions

All figures are bytes per page. Where "JPG ÷ PNG" exceeds 100%, the JPG is the heavier file on that page.

Three things stand out.

1. On dense text, JPG can be the bigger file

At 1x, page B is 142.0 KB as PNG but 161.9 KB as JPG (114%) — the opposite of the usual "JPG is lighter" assumption. JPEG is built for smooth gradients and struggles with the sharp edges of glyphs, spending data trying to preserve them. PNG, meanwhile, compresses black-on-white text extremely well because so few colours are involved. Raising the scale narrows the gap (90% at 2x, 89% at 3x), but JPG never becomes dramatically lighter on a text page.

2. On photos, JPG wins decisively

At 1x, page C is 92.0 KB as PNG against 29.7 KB as JPG (32%) — about a third of the size. For photographic content with many colours and smooth transitions, PNG's lossless compression is simply a cost. If a page carries a large photo, pick JPG.

3. Raising the scale costs less than the pixel count suggests

Going from 1x to 3x multiplies the pixel count by nine, but not the file size. As PNG we measured 5.3x for A, 6.5x for B and 2.6x for C. Enlarging a page does not add white space or flat areas — only edge detail grows. Stepping up one scale is cheaper than it looks, most obviously for photo pages (C is still only 237.1 KB at 3x). If you are unsure whether the text will be readable, going one step higher is usually the better trade.

Which scale should you pick?

• Reading on screen, sharing in chat: 2x (effective 144 dpi). At 1x, 12pt body text is only 12 px tall and falls apart when someone zooms in on a phone. That is why 2x is the default in PDF to Image.
• Pasting into a document, making thumbnails: 1x (effective 72 dpi) is enough. If the destination scales the image down anyway, exporting bigger buys nothing.
• Making fine print readable: 3x (effective 216 dpi). File size does grow honestly here, so it is more practical to export only the pages that need it at 3x.
• Printing and handing out: print work is generally expected to be 300 dpi or higher. A4 at 300 dpi means 2480 × 3508 px, about 4.2x — out of reach at 3x. When the goal is print, hand over the PDF itself.
• When it is still too heavy: change the format before you drop the scale (PNG for text pages, JPG for photo pages). If you still need it smaller, adjusting quality in Image Compressor preserves legibility better than cutting resolution.

These numbers only hold under the conditions we measured

Everything above is these three pages exported in a Chromium-based browser. Fonts, the amount of charts and line art, the subject of the photo and the browser's encoder implementation all move the absolute values. The patterns (1x = effective 72 dpi; PNG favours text pages; JPG favours photo pages; size grows far slower than the pixel count) reproduce easily, but check the KB figures against your own PDF. PDF to Image has a "Try a sample PDF" button that loads exactly the same file as page A above, so you can repeat this yourself.

FAQ

The figures in this article were measured by exporting three single-page A4 PDFs with different content through canvas.toBlob() in a Chromium-based browser (JPG at quality 0.92 on a white background — the same settings the tool uses). The 119x figure in the opening paragraph compares the bundled sample PDF (1.8 KB, 3 pages) with the 209.5 KB total of its three pages exported as PNG at 2x. As of 2026-08-10. Reading, rendering and measuring all happen inside the browser; nothing is sent to our servers.