Turning Images into a PDF: Page Size, Resolution and Why Pages Come Out Wrong

Pixels, points and the number that connects them

A PDF page is defined in points. There are 72 points to an inch, so an A4 page is about 595 by 842 points and US Letter is 612 by 792. These are physical measurements: a page describes a real sheet of paper regardless of what screen you view it on.

An image has no physical size at all. It is a grid of pixels. The only thing that gives it a real-world size is the resolution you choose to place it at, expressed in dots per inch. Place a 2400 pixel wide image across 8 inches of page and it lands at 300 DPI. Place the same image across 24 inches and it is 100 DPI and will look soft.

So DPI is not a property of your image file, whatever a metadata field may claim. It is the result of a decision about how large to draw the image on the page. Changing the DPI tag without resampling changes nothing about the pixels.

How many pixels you actually need

There is no benefit to exceeding the target. An image with more pixels than the resolution requires simply makes the PDF larger without printing any better, because the printer cannot resolve detail finer than its own output resolution. Photographic content above 300 DPI is almost always wasted bytes.

Fit, fill and stretch: three ways to place an image

Fit inside the page

The image is scaled until it fits entirely within the page, preserving its proportions. Nothing is cropped, but unless the image aspect ratio happens to match the page you get empty margins on two sides. This is the safe default and the right choice for documents, scans and anything where losing edge content would matter.

Fill the page

The image is scaled until it covers the page completely, again preserving proportions, with the overflow cropped away. No empty margins, but content at the edges is lost. Appropriate for photo books and full-bleed layouts, dangerous for anything where the edges carry information.

Stretch to the page

The image is distorted to match the page exactly. Faces get wider, circles become ovals. There is essentially no legitimate reason to use this, and when a PDF looks subtly wrong without an obvious cause, a stretch setting is often why.

A fourth option solves the mixed-orientation problem: let each page take the aspect ratio of its own image. You lose a uniform page size, which matters for printing, but nothing is cropped and nothing is padded.

Handling a folder of inconsistent images

Real image folders are rarely uniform. Portrait and landscape shots are mixed together, a couple of screenshots crept in, and the resolutions vary by a factor of five.

• Decide first whether the output is for printing or for screen. Printing demands a single consistent page size. Screen viewing does not, so per-image page sizes are often the better answer.
• If you need consistent pages, group by orientation and build two sections rather than rotating landscape images sideways, which forces the reader to turn their head or the page.
• Check the lowest-resolution image in the set against the DPI table above. That image determines the quality ceiling of the whole document.
• Name files so that lexical sorting matches the intended order. Use zero-padded numbers, because most tools sort 10 before 2.

Why the PDF is enormous, and what to do about it

A PDF built from images is essentially a container holding those images plus page instructions. Its size is the sum of the embedded images, so the controls are the same ones you would use on the images themselves.

1. Resize each image to the pixel dimensions its page actually needs. A 6000 pixel photograph on an A4 page at 300 DPI is carrying more than twice the pixels it can use.
2. Use JPEG compression for photographic pages. Lossless encoding inside a PDF produces files many times larger for no visible benefit.
3. Keep flat-colour pages, scanned text and diagrams lossless. Lossy compression on text produces ringing that makes small type noticeably harder to read.
4. Do not build at 600 DPI unless the content is genuinely line art. For photographs it quadruples the file size relative to 300 DPI and prints identically.