JPG to PDF: Turning Photos into a Document That Prints Correctly

A JPG stores pixels and says nothing about physical size. A PDF stores a page with a defined width and height in points, and places images on it at a chosen position and scale. That is why the same photo can look crisp in one PDF and blurry in another: the number of pixels is identical, but the physical area they are spread across is not. Blurriness in a printed PDF is almost always too few pixels for the page size, not a bad conversion.

The arithmetic is worth knowing. A4 is 210 by 297 millimetres, roughly 8.27 by 11.69 inches. At the 300 dots per inch that print shops expect, that is about 2480 by 3508 pixels. A phone photo at 4032 by 3024 pixels fills an A4 page comfortably. A 1000-pixel-wide image stretched to A4 is working at roughly 120 DPI, which is fine on screen and visibly soft on paper.

Decide the page size before converting, because changing it afterwards means re-laying out every image. A4 is the norm outside North America, Letter inside it, and a mismatch between the two shows up as an unexpected white strip at the edge or content clipped by the printer's unprintable border. Keep a margin of at least five millimetres on all sides for the same reason.

Orientation matters with mixed photos. A portrait photo on a landscape page leaves large empty areas; a landscape photo on a portrait page either gets shrunk to fit or gets cropped. Most converters offer fit-to-page, which preserves the whole image and leaves whitespace, or fill-the-page, which crops. For a document someone will read on paper, fitting is normally correct: a small white band is better than a headless portrait. Also check that the converter respects the EXIF rotation flag, or phone photos taken in portrait can arrive sideways.

There are two ways to get a JPG into a PDF. The first embeds the original compressed stream untouched, which is lossless from the JPG's point of view and keeps the file size roughly equal to the sum of the images. The second decodes the image and re-encodes it, usually at a chosen quality or a target resolution, which can shrink a phone-photo PDF dramatically at the cost of one more lossy generation. Neither is wrong, but you should know which one happened.

For a print order, embed the originals and let the print shop handle scaling; for an email attachment with a size limit, recompressing to around 150 DPI at quality 80 is usually indistinguishable on screen and a fraction of the size. If the photos contain readable text — receipts, whiteboards, scanned pages — keep the resolution high and avoid aggressive compression, because JPG artefacts around lettering are what makes a scanned document unreadable.

When you combine several photos, the order is the document. Most tools sort by filename, which means IMG_1002 comes before IMG_0010 unless the names are zero-padded. Rename or reorder before converting if the sequence matters, and check the first and last page of the output rather than assuming.

Mixed source formats are fine: a PDF can hold JPG, PNG and even transparent PNG images on the same page, and the transparency is preserved because the PDF page provides the background. If the document is going to be edited later, a PDF is a poor working format — keep the images and the arrangement notes, and treat the PDF as the delivered artefact.