Fitting pixels onto a page
An image has pixel dimensions; a PDF page has physical ones measured in points, at 72 to the inch. Placing an image therefore requires deciding a scale, and that decision determines print quality. A 1,200-pixel-wide photograph placed across the 8.27-inch width of A4 lands at about 145 pixels per inch — fine on screen, visibly soft in print, where 300 ppi is the usual target.
Working backwards gives a useful rule: to fill an A4 page at print quality you want roughly 2,480 × 3,508 pixels. Below about 150 ppi, upscaling will not help, because the detail is not in the file. The honest options are to accept the softness, print smaller, or rescan.
Aspect ratio is the other consideration. Photographs from a phone are typically 4:3 or 3:4, while A4 is closer to 1:1.41, so an image fitted to the page leaves margins on two sides. Filling the page instead means cropping. Neither is wrong, but stretching to fit — distorting the aspect ratio — always looks wrong, and is worth checking in the preview.
Choosing between JPEG and lossless inside the PDF
PDF can embed image data in several encodings, and the choice matters more than it appears. Photographs should stay as JPEG, and critically, a JPEG can be embedded directly without being decoded and re-encoded. That matters because every JPEG re-encode discards more information — the generation loss that makes a repeatedly re-saved image progressively blotchy around edges.
Screenshots, scanned text and line art are the opposite case. They have large flat areas and hard edges, which JPEG handles badly, producing the grey mosquito noise around black text that makes a scanned document look dirty. Lossless Flate compression keeps those crisp and often produces a smaller file as well, because flat colour compresses extremely well.
The rule of thumb: continuous-tone photograph, JPEG; anything containing text or sharp lines, lossless. Mixing them across pages in one document is entirely valid, and is what a good scanner driver does automatically.
What comes along with the image
Photographs carry EXIF metadata — the camera model, the exposure settings, the date and time, and frequently GPS coordinates precise to a few metres. Converting to PDF can carry that into the document, so a PDF assembled from holiday or property photographs may disclose exactly where and when each was taken.
EXIF also carries an orientation flag, which is why images sometimes appear rotated after conversion. The pixels are stored in one orientation and a tag tells viewers to rotate them; software that ignores the tag shows the raw orientation. If a page comes out sideways, that is the cause.
Strip metadata before conversion if the document is going anywhere public — the EXIF remover does it in one pass. To go the other direction, PDF to JPG extracts pages back out as images.