I put a very wide panorama on a page sized to the image and got a page too large error; why?

Where 14400 comes from

It is not an arbitrary cap. Acrobat and most PDF viewers will not open a page with a side longer than 14400 pt. If Images to PDF allowed a bigger page, it would create a file that opens for some people and fails or gets clipped for others. So it stops before building and tells you why: the resulting page is larger than PDF allows, so raise the DPI or choose a fixed page size.

Why the page grows when DPI drops

DPI says how many pixels fit in one inch. The lower it is, the more inches the same pixels need, so the page grows. A PDF point is 1/72 of an inch, so the page width in points is the pixels times 72 divided by the DPI. Here is what the layout function returns for an 8000 × 2000 px panorama with no margin:

• At 36 DPI: a 16000 × 4000 pt page, over the limit, so you get the error.
• At 39 DPI: still slightly over 14400, so the error stays.
• At 40 DPI: exactly 14400 × 3600 pt, the largest accepted.
• At 96 DPI: 6000 × 1500 pt, about 2117 × 529 mm.
• At 150 DPI, the suggested value: 3840 × 960 pt.

Quick rule: the lowest DPI that works is the longest side in pixels divided by 200. An 8000 px image needs at least 40 DPI, a 12000 px one needs 60. A margin is added outside the image, so allow a little extra room for it.

Three ways to output a panorama

1. Stay on Page fits the image and raise the DPI to 150. The page stays very long, in the photo's own shape, which suits on screen viewing with zoom and scroll.
2. Choose A4 with orientation Follow the image and Fit inside. The 8000 × 2000 image is drawn 785.2 × 196.3 pt on a landscape page: a complete but thin strip that prints, though small.
3. Split the panorama into sections a bit wider than they are tall, each one an image, and place them as consecutive landscape A4 pages. That gives far more detail on paper.

Does DPI change the image itself?

No. All 8000 pixels stay exactly as they are in every case, and the image is embedded without re-encoding. DPI only changes the physical size the file declares. For on screen use that size hardly matters, since viewers zoom freely. If the image is heavier than you need anyway, reduce its dimensions with the Image resizer, which shrinks the page and the file together.