JPG to PDF

A PDF is a container. When an image is placed on a page, the file's compressed bytes are embedded more or less as they are, along with instructions describing where on the page to draw them and at what scale. The PDF does not re-compress the image itself.

That has a direct consequence: the size of your PDF is essentially the sum of the images inside it. Ten camera originals of 5 MB each produce a 50 MB PDF, whatever page size you pick. This is why the images are re-encoded at a chosen quality on the way in, and why the table shows both the original and the embedded size for every page.

Three settings interact to decide what a page looks like.

Page size

The physical dimensions of the sheet. A4 is standard almost everywhere outside North America, where Letter is. Choosing "fit each page to its image" makes the sheet match the image exactly, which suits digital-only documents and photo sequences.

Orientation

Automatic gives a landscape page to a landscape image and a portrait page to a portrait one, so nothing is wasted. Fixing it is right when the document must be uniform, for example because it will be printed and bound.

Fitting

Fit to page scales the image down until it sits entirely inside the margins, which may leave bands of white on two sides. Fill page scales it until it covers the printable area, which crops whatever falls outside.

The quality number in a JPEG or WebP encoder is not a percentage of anything. It is an index into a set of quantisation tables that control how aggressively detail is discarded. The relationship to file size is steep and non-linear.

For a typical 12-megapixel phone photograph the curve looks roughly like this:

• Quality 95–100 — near-original appearance, but the file is often only 20–35% smaller. Rarely worth it for the web.
• Quality 80–90 — the sweet spot for photography you care about. Typically 50–70% smaller with no artefact you would notice at normal viewing distance.
• Quality 70–80 — the sweet spot for ordinary web images. Usually 70–85% smaller. Artefacts appear only if you zoom in on flat sky or skin.
• Quality 50–70 — visible softening and blocking in smooth areas, though thumbnails and background images survive it well.
• Below 50 — obvious blocking, ringing and colour banding. Useful only for placeholders.

Diminishing returns set in fast at the top of the range. Going from quality 100 to quality 90 usually halves the file for no perceptible change; going from 90 to 80 saves far less and costs slightly more. If you only remember one number, use 80.

A digital image has exactly one physical property: its pixel dimensions. A 3000 × 2000 photograph contains six million pixels whether its metadata claims 72 DPI or 300 DPI.

DPI is a printing instruction stored in a metadata field. It tells a printer how many of those pixels to place per inch of paper, which determines the printed size — 3000 pixels at 300 DPI is a 10-inch-wide print; the same file at 72 DPI is a 41-inch-wide print of the same six million pixels. Changing the number changes nothing about the file except a handful of bytes in a header.

So "save this at 72 DPI for the web" does not make a file smaller. Reducing pixel dimensions does. If a print shop asks for 300 DPI at A4, what they actually need is about 2480 × 3508 pixels; supply that many pixels and the DPI tag can be corrected in seconds.

Photographs of paper are the most common input here, and a few habits make an enormous difference. Shoot in even, indirect light so there is no glare band across the page and no hard shadow from your own hand. Hold the camera parallel to the paper rather than at an angle, since a trapezoid page is difficult to read and impossible to correct later without cropping. Fill the frame with the document so you are not spending megapixels on the desk around it.

For pages of text, quality 80 and a longest edge of about 2000 pixels keeps the writing sharp while producing a PDF that will actually send by email. Going higher rarely improves legibility and multiplies the file size.