Image to PDF without losing quality: what actually touches your pixels

Convert without loss

Drop the pictures, pick a paper, click Make the PDF. There is no quality setting to get wrong, because there is no quality setting: nothing in the conversion throws visible detail away. The result lists what happened to each kind of file, for example “12 JPEGs embedded as they are, no re-encoding” or “3 pictures stored losslessly”.

The core fact: a JPG goes in as it is

A PDF can hold a JPEG directly: its DCTDecode filter is JPEG compression. So for an ordinary (baseline) JPG, the converter does not decode the photo at all. It copies the compressed picture data into the PDF and writes a page around it.

We checked this with a 12 megapixel test photo (4032 by 3024 pixels, saved by a camera-style encoder at quality 92). The JPG was 2,537,619 bytes; the PDF with it on an A4 page was 2,538,557 bytes. The 938 extra bytes are the page, the file’s table of contents and its trailer. The picture data inside is byte for byte the original; the test suite checks this on every change.

One thing is left out on purpose: the camera’s metadata. A phone writes the GPS position, the phone model and the time into each JPG. Those segments are dropped, the picture data is not touched, and the result says how many photos were cleaned. What PDF metadata reveals explains why that matters.

Every format, and what happens to it

The HEIC row and the WebP and AVIF photo row are the only ones that re-encode. A lossless copy would have been the alternative, and it is huge: on our 12 megapixel test picture, quality 90 came to 3.1 MB and lossless to 21.8 MB, 7 times as much for a difference nobody sees in a photo that was already lossy. A graphic with few colours, such as a logo or a diagram saved as WebP, stays lossless, because JPEG would blur its edges. HEIC to PDF on Windows has more on that format.

Page size is display, not pixels

Choosing A4, US Letter or a margin changes how large the picture shows on the page, never how many pixels it has. The PDF stores the whole picture and an instruction to draw it at a given size; a viewer zooms, a printer prints at the size given, and the full detail is there for both.

So “will A4 make my photo worse” has a measured answer: no. The 4032 by 3024 test photo on a landscape A4 page fills 280 by 210 mm, which works out to about 366 pixels per inch. With a 10 mm margin it is 404 pixels per inch, because the same pixels cover a smaller area. Either is plenty for a printed page. “Same as the picture” makes the page the size the photo’s own resolution note says, which changes nothing about the pixels either. Picking the page size has the full table.

Only Fill removes anything: it covers the page and cuts off what sticks out, about 6 % of a 4:3 photo on A4. That is cropping, not blur, and Fit (the default) avoids it.

Where loss really comes from

If a picture PDF looks soft, it is almost always one of these:

1. The source was soft. A zoomed phone photo, a shot in dim light, a picture a chat app recompressed before it reached you. The PDF shows exactly what it was given.
2. A detour through JPG. Screenshots, diagrams and scans of text saved as JPG get fuzzy edges around letters. Keep them as PNG the whole way; screenshots to PDF shows the difference in numbers.
3. Compressing the PDF afterwards. That is the real trade. Compress PDF re-encodes pictures at a lower resolution and quality: Print at 300 dpi and quality 85, Web at 150 dpi and 75, Email at 110 dpi and 65. In our test, 20 phone photos made a 60.7 MB PDF; Web took it to 4.8 MB and Email to 2.2 MB.

The order matters: convert first at full quality, keep that file, and compress a copy for sending. You can always make a smaller copy from the full one, never the other way round.

What lossless conversion cannot do

• It cannot add sharpness. A blurry photo stays blurry; no converter has the detail the camera never caught.
• It does not make files small. Keeping every pixel means the PDF is about as large as the pictures together. If the file must fit an email or a portal, compression is the price, and the guide to making a PDF smaller shows how much each step costs.
• A copy of a lossy picture is still lossy. A JPG or a WebP that was already compressed keeps the artefacts it had; copying a JPG as it is adds none, and the quality 90 pass on a WebP or AVIF photo adds almost none.

All of this happens on your device, so the original pictures are never uploaded anywhere. For every format and option in one place, see Pictures to PDF.