JPEG quality and PDF file size

2026-10-04

A JPG is already compressed. When your phone or scanner saved it, some fine detail was traded for a smaller file. Every time a JPEG is opened and saved again, a little more is lost. So the real question for a JPG to PDF tool is simple: does it save your photos again, or not?

JPG to PDF has two options in Page setup: Quality first, the default, and Smaller file. Details under the options gives a short version of what follows.

Quality first: most JPEGs go in directly

With Quality first, most JPG photos are not re-encoded. Ordinary JPEGs — 8-bit, baseline or progressive, in colour or greyscale — are placed into the PDF with their compressed image data copied across. That covers the photos phones and most scanners produce. The page holds the same pixels at the same compression as your file; only hidden details such as location and camera information are left out.

This also means the PDF cannot look better than the JPG. If a photo was heavily compressed — a picture that went through a messaging app, for example — its blocky patches stay.

The JPEGs that are converted

This version of JPG to PDF does not place every JPEG directly. CMYK files (common from print and design software), 12-bit files and rare variants such as arithmetic-coded, lossless or hierarchical JPEGs are converted instead: when your browser can decode them, they are saved again as a high-quality JPEG and placed on the page, and their card says Re-encoded as JPG. If your browser can’t decode such a file, its card shows an error and the file doesn’t become a page.

An image that has to be decoded is also limited in size. Above about 16.7 million pixels it is scaled down to fit the device limit, and its card says Scaled down to fit the device limit. JPEGs that go in directly are not affected by this limit: with Quality first they are not scaled down, whatever their pixel size. How many very large photos one PDF can hold still depends on your device’s memory.

Why your PDF is so big

The PDF is roughly the sum of the JPEG data inside it. A phone photo is often several megabytes, so a PDF of 30 photos can easily be larger than an email or an upload form allows. With Quality first that is expected: the PDF is about as large as the photos themselves, minus their hidden details.

Smaller file

Smaller file is the option for size limits:

* except decoded images above the device limit.

† The candidate is used only if it makes that photo’s data smaller; otherwise the photo keeps its Quality first version, pixel size included.

‡ A very long or wide image keeps at least 600 px on its short side instead; one that is already narrower keeps its pixel size.

For each photo, Smaller file prepares a candidate: at most 2000 px on the long side, saved again as a JPEG at a lower quality. A very long or wide image, such as a scrolling screenshot or a panorama, keeps at least 600 px on its short side instead, so its text stays readable. The candidate is used only if it really makes the file smaller; otherwise the photo’s Quality first version is kept, so a photo with a long side above 2000 px can stay that size. How much the PDF shrinks depends entirely on your photos, so there is no fixed size to expect.

On an A4 page, a 2000 px candidate is about 171 dots per inch — enough to read ordinary text in a photographed document. Fine print and small details can look softer when you zoom in.

See the real size

After you tap Save PDF, the strip at the bottom shows the number of pages and the real file size. To compare, save once with Quality first, switch to Smaller file and save again. Download again only gives another copy of the last PDF.

Which one to choose

• Keep Quality first for photos you will print or zoom into, scanned documents with small print, and anything you archive.
• Choose Smaller file when a size limit matters more than the last bit of detail.

How large your photos will look on the page is covered in page size for JPG photos.