When a PDF beats a folder of images
Many places that accept documents want a single PDF, not a pile of photos. Common situations:
- Sending scanned or photographed paperwork to a bank, landlord, visa centre or HR department that asks for "one PDF file".
- Turning phone photos of a multi-page contract, receipt set or handwritten notes into something that opens the same way on every device.
- Bundling screenshots for a bug report or a step-by-step guide, so the order is fixed and nobody has to open twelve attachments.
A PDF keeps the page order, prints predictably and is accepted by almost every document upload form.
How images are placed on pages
The conversion runs entirely in your browser using pdf-lib, a JavaScript library for building PDF files. Each image becomes exactly one page, in the order shown in the list.
PDF pages are measured in points, where 72 points equal one inch. The tool treats one image pixel as one point, then applies these rules:
- A4 pages are 595×842 points (210×297 mm). Letter pages are 612×792 points (8.5×11 inches). The orientation setting turns them sideways for landscape.
- The margin slider runs from 0 to 72 points. The default of 36 points is half an inch, about 12.7 mm, on every side.
- The image is centred in the area inside the margins and shrunk if it does not fit, keeping its proportions. It is never enlarged.
Two consequences are worth knowing. A typical 12 MP phone photo (about 4000×3000 px) is far larger than an A4 page, so it is scaled down to fill the width or height inside the margins; the full resolution stays in the file and is visible when zooming in. A small image, such as a 400×300 px screenshot, is placed at its natural size of roughly 14×10.5 cm and floats in the middle of the page with white space around it.
Fit to image
With Fit to image, every page takes the exact size of its image plus the margins, and the orientation setting is ignored. Screenshots look best this way. For photos the page becomes very large: a 4000×3000 px photo gives a page of about 144×108 cm with the default margin. That is fine on screen, but choose A4 or Letter if the PDF will be printed, so the printer does not have to guess how to scale it.
Image quality and file size
JPG files are embedded into the PDF as they are, without being decoded and compressed again. There is no generation loss: the picture inside the PDF is byte-for-byte the photo you added. PNG files are stored losslessly and keep their transparency.
This also means the PDF is roughly as large as all the images combined. Ten photos at 4 MB each produce a PDF of about 40 MB, which is over the 10 to 25 MB limit that many email services and upload portals enforce. To keep the result small:
- Reduce the images before converting. Around 2000 px on the long side is plenty for an A4 page viewed on screen or printed at office quality.
- Prefer JPG over PNG for photos and scans. A photographed page saved as PNG can be five times larger than the same page as JPG.
- If the PDF is already built, compress the PDF to bring it under the limit.
Common problems and their fixes
A photo still lies on its side. Phones save the picture in sensor orientation and record the intended turn as an EXIF tag; the converter reads that tag and turns JPG pages accordingly. Images that were edited or exported without the tag, and PNG screenshots, are placed exactly as stored, so rotate them first if they come out wrong.
Pages are in the wrong order. Pages follow the list order. The list has no drag-to-reorder, so remove misplaced files and add them again in the right sequence. Naming files 01.jpg, 02.jpg and so on before selecting them often avoids the problem in the first place.
Landscape and portrait photos mixed in one document. The orientation choice applies to all pages, so a landscape photo on a portrait A4 page is shrunk to fit the width and leaves empty space above and below. Either accept that, use Fit to image so each page follows its own image, or make two PDFs and join them with the merge tool.
Scans look grey or uneven. The tool does not alter the image, so shoot in daylight, hold the phone parallel to the page and crop away the table around the sheet.
Each image can be up to 50 MB. Nothing is uploaded: the images and the finished PDF stay on your device.