How images are embedded in PDFs
When you convert an image to PDF, the image data is embedded directly inside the PDF file. JPEG images can be embedded as-is (the PDF spec natively supports JPEG streams), keeping file size close to the original. PNG images are typically re-compressed or converted to JPEG during embedding — if your PNG has transparency, the transparent areas become white (PDF pages have no built-in transparency at the page level).
The PDF page size and the image size are separate: the page is an A4 or Letter frame, and the image is scaled to fit within it. If your image is wider than tall, landscape orientation preserves more image area than portrait. Most converters detect aspect ratio automatically.
DPI and print quality
DPI (dots per inch) determines print sharpness. A screen image at 72–96 DPI looks fine on a monitor but prints blurry — at A4 size (8.27 × 11.69 inches), 72 DPI yields only 595 × 842 pixels. For sharp print output, you need the source image to be at least 1240 × 1754 pixels (150 DPI on A4) or ideally 2480 × 3508 pixels (300 DPI — professional print standard).
A web screenshot at 1920 × 1080 pixels embedded on an A4 PDF page prints at roughly 231 DPI — acceptable for office documents but below the 300 DPI threshold for photographic print quality. If the output will be professionally printed, start with the highest resolution source image available.
Combining multiple images into one PDF
The most common use case for image-to-PDF conversion is scanning: photographing multiple pages of a document and combining them into a single PDF file. Each image becomes one page. The page order matches the order you upload or select the images — most tools allow drag-to-reorder before generating. For smartphone scans, dedicated scanning apps (Adobe Scan, Microsoft Lens) handle perspective correction and contrast enhancement before conversion, producing cleaner results than a direct photo-to-PDF.
When to combine images into a PDF — common use cases
| Use case | Images to combine | Recommended settings |
|---|---|---|
| Submit scanned documents (passport, ID, utility bill) | JPG scans of each page | A4 portrait, fit image to page, no margins |
| Send multiple screenshots as one file | PNG screenshots | Letter size or A4, default margins for readability |
| Combine product photos for a catalogue | High-res JPG product images | A4 landscape, tight margins, one image per page |
| Submit assignment scans to a school/university portal | JPG photos of handwritten pages | A4 portrait, greyscale if supported, max quality |
| Bundle receipt photos for expense report | Phone camera JPGs | Multiple per page or one per page depending on size |
| Create a photo book or storyboard PDF | JPG or PNG images | Custom page size matching your print specs |
| Archive printed invoices (scanned) | JPG scans | A4 portrait, lossless quality for archival |