JPG to PDF: combine images into one PDF

What this tool does

Each picture becomes one page of a new PDF, in the order of the list. You choose the page size (A4, US Letter or the size of each picture), the orientation, the margin and how the pictures are stored.

Original quality is preselected. It copies the compressed data of JPG pictures into the PDF unchanged, so the pictures lose nothing a second time, and stores PNG pictures without loss. The other choices write every picture as a new JPG, which usually makes the PDF smaller.

The PDF contains pictures of the pages. Text in the pictures cannot be selected or searched, because the tool does not recognise text.

How to use it

1. Choose the pictures, or drop them on the marked area. You can add more afterwards.
2. Put them in order with the arrow buttons of each row.
3. Choose page size, orientation and margin. Keep Original (no re-compression) for the best quality, or choose a JPG quality for a smaller file.
4. Start. Each row says how its picture was stored.
5. Download the PDF.

The result is named after the first picture of the list, with -images added.

How pictures are stored in the PDF

PDF can hold JPG data directly: a reader decodes it like an image viewer does (ISO 32000-1, section 7.4.8, the DCTDecode filter). With Original, a JPG picture is therefore not decoded and encoded again. Its data goes into the PDF byte for byte; only the metadata segments are left out: Exif (camera, time, GPS position), XMP, IPTC and comments. A colour profile in the file, such as the Display P3 profile of recent phones, is attached to the picture, so that PDF readers can show its colours as intended.

Phones often store a photo turned and record in Exif how to show it. The tool reads that orientation and turns the picture on the page instead of changing its data, so the page looks the way a gallery shows the photo.

PNG pictures are stored with lossless compression, and transparent areas stay transparent. PNG pictures with 16 bits per channel are reduced to 8 bits. WebP pictures, animated PNG and rare kinds of JPG (12-bit, lossless or arithmetic-coded JPG, CMYK without the Adobe marker) cannot go in as they are; they are decoded and stored as PNG pixels, which adds no further loss.

High JPG, Balanced JPG and Small JPG write every picture as a new JPG with quality 92, 80 or 60. Transparent areas then become white, and metadata is not copied.

How pictures are placed on the page

A picture pixel is taken as 1/96 of an inch, the size of a pixel on web pages. PDF measures in points of 1/72 of an inch, so one pixel is 0.75 points. A picture of 960 × 480 pixels is 720 × 360 points at its natural size, which is 10 × 5 inches.

• Same as each image: the page gets the natural size of the picture plus the margin.
• A4 or US Letter: a picture that is larger than the area inside the margins is scaled down until it fits, keeping its proportions. A smaller picture keeps its natural size. The picture is centred on the page.
• Orientation Follow each image turns the sheet to landscape for pictures that are wider than high.

Scaling on the page does not remove pixels. The picture is stored with all its pixels and only shown smaller, so a scan keeps its resolution for printing.

Worked example

A scan of an A4 sheet at 300 pixels per inch has 2480 × 3508 pixels. Its natural size is 2480 × 0.75 = 1860 points wide and 3508 × 0.75 = 2631 points high, far larger than an A4 page of about 595 × 842 points. On an A4 page without margin, the scan is scaled by the smaller of 595.28 ÷ 1860 and 841.89 ÷ 2631, which is about 0.32, and fills the page. All 2480 pixels are spread over the page width of 8.27 inches, which is again 300 pixels per inch.

File size: five camera photos between 1668 × 2500 and 4288 × 2848 pixels, 10,127,078 bytes together, were put on A4 pages in Chrome. Original gave a PDF of 10,065,990 bytes in a quarter of a second, about 60 KB less than the photos because their metadata was left out. High JPG gave 8,785,479 bytes, Balanced JPG 4,542,464 and Small JPG 2,652,707.

High JPG was smaller than Original because these cameras had saved their photos at a quality of about 94 to 95 on the usual 1 to 100 scale (estimated from the quantization tables in the files). Original is the choice when nothing may be lost; a JPG quality is the choice when the file must be small.

Common mistakes

• Choosing Small JPG for scans of text. Fine print gets smeared edges at low quality. Keep Original or use High for documents.
• Expecting Original to give the smallest file. It keeps the pictures as they are; a JPG quality is what makes the PDF smaller.
• Expecting selectable text. The PDF contains pictures only.
• Adding hundreds of large photos on a phone. All pictures are held in memory until the PDF is written. Use fewer pictures per run or a computer.