Turning pictures into a PDF feels like it should be the simplest thing a PDF tool does. Put image in, get page out. And it is simple — right up to the moment someone asks why the pages are a strange size, or why the file is bigger than the pictures were, or why the text in the scan cannot be searched.
All three have the same answer, and it fits in one sentence: a PDF page is measured in points, an image is measured in pixels, and nothing automatically converts between them.
Pixels are not a size
An image 1600 pixels wide has no physical size. It is 1600 samples of colour. How big it should be printed or displayed is a separate question, and the answer is a resolution: at 300 DPI those 1600 pixels are 5.3 inches; at 72 DPI they are 22.2 inches. Same image, same pixels, four times the paper.
A PDF page, by contrast, is measured in points, and a point is exactly 1/72 of an inch. A4 is 595 × 842 points. US Letter is 612 × 792.
So when an image becomes a page, something has to decide how many points wide that image should be. The simplest rule — and the one our JPG to PDF currently uses — is one pixel to one point.
We measured it. A 1600 × 1200 pixel PNG became a page of exactly 1600 × 1200 points: 22.2 inches by 16.7 inches, which is roughly A1. Not a mistake, just the arithmetic. One pixel became one point, and one point is 1/72 inch, so the page is the image at 72 DPI.
That is worth knowing because it explains the two things people notice. On screen it looks perfectly normal, because a screen renders a page at whatever zoom fits the window. Sent to a printer it is enormous, and the printer will either scale it down to fit the paper or print it across several sheets.
The file usually gets smaller, not bigger
The same run: a 200 KB PNG became a 55 KB PDF.
That surprises people who expect a wrapper to add weight. PNG is lossless, which is excellent for screenshots and line art and wasteful for photographs; placing the image in a PDF re-encodes it as JPEG, which throws away detail the eye does not miss. For a photograph that is a good trade and the file drops.
Run it the other way with PDF to JPG and the same image came back out at 185 KB — bigger than the 55 KB it occupied inside the PDF, because it has been decoded and re-encoded again as a standalone JPEG at high quality.
The number worth taking from this is not any of those three. It is that each conversion re-encodes. A photograph that goes image → PDF → image has been through JPEG twice, and JPEG is lossy every time. Once is invisible. Five round trips is not, and the damage never comes back.
A picture of text is not text
This catches people out constantly, and it is worth being blunt about.
When a photograph of a document becomes a PDF page, the PDF contains a photograph. It does not contain the words. Ctrl+F finds nothing, you cannot select a sentence to quote, and a screen reader gets silence. The page looks like a document and behaves like a picture, because it is one.
Adding a text layer is a separate operation: OCR recognises the characters in the image and writes them behind it, which is what makes the page searchable. It is a recognition pass rather than the original text, so it is a best guess — but it is the difference between a findable document and a flat image.
If you are converting photographs of paper rather than photographs of things, assume you want OCR afterwards.
Practical consequences
Order is set before you convert, not after. Images go onto pages in the order shown on the page, which is why they are draggable. Filenames sort in ways that surprise — IMG_9.jpg lands after IMG_10.jpg in a plain alphabetical sort — so check the sequence rather than assuming the camera’s numbering survived.
Shrink before you convert, not after. If the pictures are coming off a modern phone at 12 megapixels and the document is for email, resizing the images first gives a far better result than compressing the PDF afterwards. Compression works by reducing image resolution, so you end up at the same place having re-encoded everything twice.
Mixed orientations stay mixed. Portrait and landscape photographs produce portrait and landscape pages. That is correct behaviour and it looks untidy; if a document needs one consistent paper size, build it in a word processor and use Word to PDF, which is the tool that fits content to a sheet.
Screenshots are the opposite case. A screenshot is captured at screen resolution, roughly 72 to 96 pixels per inch. One pixel to one point renders it at close to its natural size and it looks right. The same screenshot forced onto A4 and enlarged looks soft, because there were never enough pixels to fill that much paper.
The rule of thumb
Decide what the document is for before you convert, because the answer changes what matters.
For reading on screen, the defaults are fine and the page size is irrelevant. For printing, work backwards from the paper: 300 DPI on A4 needs an image about 2,480 pixels wide, and no tool can add pixels that were never captured. For archiving or searching, convert, then run OCR, and check that selecting a line of text actually highlights it.
You can put your own images through JPG to PDF and open the result’s page size in any reader — the arithmetic above will be sitting right there.
