Watch the page size if your PNGs are small. On Letter or A4 pages every image is scaled to fill the page, so a 256-pixel icon is blown up to the full page width and looks soft. “Same as image” keeps each image at the size a browser shows it, 96 pixels per inch, so that icon gets a page of about 2.7 inches square.
A logo and a photo in one PDF
A 1 KB transparent logo (256 × 256 pixels) and a 442 KB photo (600 × 400) made a two-page PDF of 445 KB in about a fifth of a second, with the default Letter size and Auto orientation. The square logo got a portrait page and the wide photo a landscape one. The PDF is named after the first image in the list, here benchmo-logo-transparent.pdf, so put the image whose name you want to keep at the top.
Page size, orientation and margin
- Same as image for screenshots that will be read on a screen. This is what we would choose for a support request: the text stays the size it was when you took the screenshot.
- Letter or A4 for printing. Every image is fitted to the page and centered.
- Orientation: Auto gives wide images a landscape page and tall ones a portrait page. Portrait or Landscape forces one orientation for all pages.
- Margin adds white space around each image: None, Small (10 mm) or Large (20 mm). With “Same as image” the margin is added around the image, so the page grows.
How the image data goes in
PDF has no PNG format of its own, but it can store the same kind of lossless compression. For ordinary PNGs without transparency, the compressed image data is copied straight into the PDF without being decoded, which is why the PDF above was only 2 KB larger than the two images. PNGs with transparency are compressed again, still without loss, with the transparent parts kept as a mask. For photos, JPG to PDF gives the same result with JPG files, and if you already have PDFs to add, Merge PDF joins them afterwards.