Turning a photo into a PDF without bloating it: JPG or PNG, our measurements

FileXvert creates an A4 page, oriented like the image (portrait for a vertical photo, landscape for a horizontal one), with a 24-point margin, about 8.5 mm. The image is centred and scaled down to fit the page. It is never enlarged beyond its natural size at 96 pixels per inch: a small image stretched over the whole sheet would print blurry.

The key point lies elsewhere: a JPEG file is embedded in the PDF as it is. Its bytes are copied without being decoded or recompressed, so the image in the PDF is exactly the photo. A PNG, on the other hand, cannot be stored as is in a PDF: its pixels are decoded and then recompressed with the PDF format's own compression method. WebP, which the PDF library used cannot embed, is first re-encoded as a high-quality JPEG by your browser.

We converted three real images with FileXvert's conversion code: two JPEG photos, and a 38.5-megapixel smartphone photo saved as PNG. For the latter, we also measured the detour through a prior PNG → JPG conversion, with FileXvert's JPEG setting (FFmpeg's highest quality, -q:v 2).

For a JPEG, the PDF adds only 0.9 KB: the page description and some information about the document. For the PNG, the PDF is 31% heavier than the image, even though it contains exactly the same pixels: the PDF's compression is less efficient here than PNG's. Going through JPG first, the same document weighs 4.74 MB, eight times less.

PNG is a lossless format, ideal for a screenshot, a logo or a diagram: large flat areas of colour, sharp edges, text. For a photo, it faithfully preserves every nuance of the sensor's noise, which its compression cannot reduce. JPEG, designed for photography, discards precisely those details the eye cannot tell apart. Our guide to image formats details this gap on other examples.

If your photo is a PNG, because it comes from a capture, an editing program or a phone set up that way, convert it to JPG first, then the JPG to PDF. At the quality FileXvert uses, the difference is not visible to the eye on a photo, and the PDF is far easier to send.

Keep PNG, on the other hand, for a screenshot of a document or a black-and-white scan: the text stays perfectly sharp, without the slight halos JPEG creates around letters.

Good-quality printing requires about 300 dots per inch. A full A4 page at 300 ppi is roughly 2480 × 3508 pixels, or 8.7 megapixels. Our 38.5-megapixel photo is printed here at 839 ppi: almost three times more detail than a printer can reproduce, and as much useless weight.

Conversely, the second JPEG image, printed at 119 ppi, will remain readable but slightly soft on paper. For a document that will only be read on screen, that level is more than enough.

To get a light, sharp PDF: