Photos to PDF: page size, DPI and why your scan looks like a phone picture

A JPEG has no concept of paper. It is a grid of pixels with an optional tag saying what size it would like to be printed at. A PDF has real pages measured in millimetres or inches. Converting means deciding how many of your pixels map onto each page, and that decision is called the effective resolution, measured in dots per inch.

The converter lays out one image per page in the order you choose. This site decodes every image and re-encodes it as JPEG at the selected quality, including source JPEGs and PNGs. Transparent areas are composited onto white. A second JPEG encoding may introduce artefacts, so inspect the PDF and keep the original image.

There are three sensible choices, and the right one depends on what happens next.

• A4 pages (210 × 297 mm). Standard for anything that will be printed, filed or posted. Text documents photographed on A4 belong here. Portrait is the default for documents; landscape for wide tables and certificates printed in landscape.
• Fit-to-image pages. Each page is exactly the size of the image, so nothing is padded with white margins. Ideal for a folder of ID copies, tickets, or receipts that will only ever be read on screen. It is also the smallest option, because no empty page area is created.
• Another standard paper size. The workbench also offers Letter and A5, but not an arbitrary custom page size. If an authority requires another specific page size, use a suitable editor and check the finished page dimensions.

A 12-megapixel phone photograph is around 4000 × 3000 pixels. Fitted to an A4 page that is roughly 480 DPI — comfortably beyond what a home printer can resolve and, more importantly, three to five times more data than a portal needs. This is the single biggest reason a photo-to-PDF conversion produces a 14 MB file that a form will not accept.

For a document you will read on screen, 150 DPI is enough: at that resolution, 8-point text is legible. For something that will be printed and filed, 200 to 300 DPI. For a form that will be scanned, OCR-processed or sent to a government office, err on the side of 200 DPI with lower JPEG quality rather than 600 DPI with maximum quality — the first stays under most size limits and loses nothing a clerk can see.

If you only need the file to be under a cap, the order of sacrifice matters: reduce quality first, resolution second, and never crop the margins of a document you did not photograph fully.

Phone photographs can appear sideways if orientation information is interpreted differently. This workbench has no individual image-rotation control, so rotate a source separately if necessary and check the downloaded PDF.

Order matters more than people expect. Filenames sort alphabetically, which means IMG_10.jpg comes before IMG_2.jpg. If your document pages are numbered, rename them with zero-padded numbers (page-01, page-02) before converting, or supply the images in explicit order. A shuffled bank statement is worse than no statement.

Margins do two jobs: they stop a printer from cropping the last two millimetres of your text, and they make a document look deliberate. A 10 mm margin on A4 is enough for both. If you are fitting full-page scans, no margin is required as long as the scan includes the whole sheet.

An image-based PDF of ten A4 pages at 300 DPI in colour is around 25 to 40 MB. The same ten pages at 150 DPI in grayscale with moderate JPEG quality is well under 3 MB, and for text on white paper the visual difference in on-screen reading is close to nil.

Colour, resolution and JPEG quality can affect size, but their effect depends on the image. This converter offers a JPEG-quality control; it does not provide a grayscale option or a projected file size. If needed, prepare a lower-resolution source separately, then inspect small text, stamps and signatures in the resulting PDF.

It does not make your phone photo look like a flatbed scan. Perspective distortion, uneven lighting and shadows are properties of the capture, and fixing them properly requires deskewing and illumination correction — a different class of tool. If your photograph is at an angle, the cleanest fix is to take another one straight above the page with even light.

It also does not add a text layer. A PDF made from images contains no searchable text and no OCR, which means a portal that tries to extract your certificate number automatically will fail. If a form requires machine-readable text, you need a document that was digital to begin with, or a separate OCR step whose output you should check character by character before submitting.