PDF to Image and Back: DPI, Formats and File Size Explained

Converting between PDFs and images sounds like a single click, and often it is. But the settings you choose decide whether the result is crisp or blurry, a few hundred kilobytes or dozens of megabytes, and whether a scanned receipt ends up readable or not. Two ideas explain almost everything: resolution, usually expressed in DPI, and the image format. Once those are clear, choosing the right options takes seconds.

What DPI actually means

A PDF page has a physical size, an A4 sheet is 210 by 297 mm, but an image only has pixels. DPI, dots per inch, is the bridge between the two: it says how many pixels are created for every inch of the page. The higher the DPI, the more pixels, the more detail and the bigger the file.

For an A4 page the numbers look like this:

• 72 DPI: about 595 × 842 pixels. Enough for a small preview or a thumbnail; text becomes hard to read when you zoom in.
• 150 DPI: about 1240 × 1754 pixels. A good default for viewing on screens, sharing in messaging apps, presentations and most web uses. Normal text stays sharp.
• 300 DPI: about 2480 × 3508 pixels, roughly 8.7 megapixels. The usual standard for printing and for archiving documents where small print and fine lines matter.

Doubling the DPI quadruples the number of pixels, because both width and height double. That is why a 300 DPI image is roughly four times the size of a 150 DPI one. Going above 300 is rarely useful for documents: it shows detail that was never on the page and makes files unwieldy. Exceptions are tiny print, detailed technical drawings or when you plan to crop a small area and enlarge it.

JPG, PNG or WEBP?

Each common format compresses images in a different way, and that matters more than people expect.

• JPG uses lossy compression tuned for photographs. It makes small files from photos and scans with gradual tones, but it can leave faint blotches around sharp edges such as text, especially at low quality. It does not support transparency.
• PNG is lossless: every pixel is kept exactly. It is excellent for screenshots, diagrams, text-heavy pages and logos, and it supports transparent backgrounds. On photographs, PNG files get very large.
• WEBP is a newer web format that can work in either lossy or lossless mode and usually produces smaller files than JPG or PNG at similar quality. Modern browsers support it well, but some older software and print workflows do not.

vupdf's PDF to JPG tool produces JPG images in full color at high quality, which is the most widely compatible choice: every phone, computer, email client and social network can open them. If you specifically need PNG or WEBP, for example for a transparent logo, you can convert the JPG afterwards in an image editor, but for most documents JPG at a suitable resolution is the practical answer.

Turning PDF pages into images

In vupdf you choose the resolution, Small (72 DPI), Standard (150 DPI), High (300 DPI), or a custom value between 36 and 600 DPI, and the tool shows the pixel size the first page will have. Very large pages are capped at 100 megapixels so the images stay usable. You can convert all pages, only odd or even pages, or a range such as 1-3, 5, 8-10.

For multi-page documents you can choose between separate images, one per page, downloaded together in a ZIP file, or a single tall image with all pages stacked. The tall image is handy for sharing a short document in a chat app where people scroll naturally; separate images are better when each page is used on its own, such as slides for social media or pages inserted into another document.

Some practical choices:

• Sharing a page in a messaging app or on a website: 150 DPI.
• Inserting a page into a presentation shown on a large screen: 150 to 200 DPI.
• Printing the image or sending it to someone who will print it: 300 DPI.
• Making small preview thumbnails: 72 DPI.

Turning photos and scans into a PDF

The other direction, combining images into a PDF, is how most people digitize receipts, ID copies, handwritten notes and multi-page paper documents photographed with a phone. vupdf's JPG to PDF tool accepts JPG, PNG, WEBP, GIF, BMP, AVIF and TIFF images, up to 20 files of up to 20 MB each. You can drag them into the right order, rotate any that came out sideways and rename them before converting.

Page size

Pick a standard paper size when the PDF may be printed or submitted to an institution: A4 in most of the world, US Letter or Legal in North America; A3 and A5 are also available. Choose “Same as image” when the PDF is only for viewing and you want each page to match its picture exactly, with nothing shrunk or cropped. Orientation can be portrait, landscape or automatic, where each page follows the shape of its image, which is useful when you mix vertical and horizontal photos.

Fitting and margins

Two fit options decide how a photo sits on a standard page. Fit whole picture keeps the entire image visible and leaves empty space where the proportions differ; this is the safe choice for documents, because nothing gets cut off. Fill the page enlarges the image until it covers the whole page and crops the edges that do not fit; it looks good for photos but can cut off the edge of a document.

Margins add a white frame, or a gray or black one if you change the page color. No margin suits photos and full-page scans. A small or large margin makes printed pages look tidier and keeps content away from areas some printers cannot reach. You can also set a custom margin in millimeters.

Output

You can create one PDF with all images as pages, the usual choice for a multi-page document, or a separate PDF per image, downloaded in a ZIP file, which is useful when each image is its own document, such as a set of receipts that will be submitted individually.

Getting better results from phone photos

• Shoot in good, even light, ideally daylight, and avoid the shadow of your phone falling on the page.
• Hold the phone parallel to the paper; strong angles distort the text and make edges trapezoid-shaped.
• Fill the frame with the document and place it on a contrasting, plain background.
• Crop away the table and your fingers before converting, using your phone's photo editor.
• Check that small print is readable in the photo before you put it into a PDF; a PDF cannot add detail the photo does not have.

In short

Use 150 DPI for screens and 300 DPI for print, pick JPG for photos and broad compatibility, and remember that PNG is better for sharp graphics and transparency. When making a PDF from images, choose a real paper size if it will be printed, fit the whole picture for documents, and add a margin for a cleaner page. With those few choices, conversions come out sharp and reasonably sized every time.