Converting PDF pages to images at the right resolution
What DPI means for a PDF page, the pixel size of A4 and Letter at each setting, how to meet a form's rule, and why a very large page comes out at a lower DPI.
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In short
- 150 DPI suits sharing and most upload forms, and makes an A4 page 1240 × 1754 pixels. Use 200 when small print must stay sharp and 300 for printing.
- To meet a pixel rule, work out DPI = pixels × 72 ÷ points and take the next setting up. 2,000 pixels on the long side of A4 needs just over 171 DPI, so choose 200.
- Doubling the DPI quadruples the pixels. On the test page with a photo, the JPG grew from 87 KB at 150 DPI to 240 KB at 300 DPI.
- A page drawn below the DPI you chose was over a canvas limit, 16,777,216 pixels in all or 16,384 pixels on a side. Its JPG records the DPI actually used, so it still prints at the page's real size.
- Convert more than 500 pages in runs such as 1-500, 501-1000 and 1001-. Download each run's ZIP before the next run, because a new run clears the last one's JPGs and every ZIP gets the same name.
- The JPGs are ordinary images with no password, so images made from a protected PDF need the same care as the PDF.
A portal wants your marksheet, salary slip or signed letter as JPG images, and all you have is the PDF. The first setting that decides whether those images are readable, printable and small enough to upload is the resolution, measured in DPI. JPEG quality comes second.
What DPI means for a PDF page
A PDF page has no pixels. Its size is stored in points, a point is 1/72 of an inch, and an A4 page measures 595.28 × 841.89 points. Pixels only exist once something draws the page.
DPI (dots per inch) is how many pixels the converter draws for each inch of paper:
pixels = points × DPI ÷ 72
For A4 at 150 DPI the width is 595.28 × 150 ÷ 72 = 1,240.2, which rounds to 1,240 pixels.
A higher DPI draws the same page with more pixels; it does not make the page bigger. Typed text and vector drawings gain real sharpness because they are redrawn from scratch at the new scale. A scanned page is already a picture with its own resolution, and drawing it at a higher DPI enlarges that picture without adding detail the scanner never captured.
Pixel sizes of A4, Letter and Legal
Computed with the converter's own sizing code:
| Page | 72 DPI | 150 DPI | 200 DPI | 300 DPI |
|---|---|---|---|---|
| A4 (595.28 × 841.89 pt) | 595 × 842 | 1240 × 1754 | 1654 × 2339 | 2480 × 3508 |
| Letter (612 × 792 pt) | 612 × 792 | 1275 × 1650 | 1700 × 2200 | 2550 × 3300 |
| Legal (612 × 1008 pt) | 612 × 1008 | 1275 × 2100 | 1700 × 2800 | 2550 × 4200 |
Going the other way gives the printed size: cm = pixels ÷ DPI × 2.54. The 150 DPI A4 image is 1,240 ÷ 150 × 2.54 = 21.0 cm wide and 1,754 ÷ 150 × 2.54 = 29.7 cm tall: the A4 sheet again.
Other software can make that round trip on its own because each JPG records its DPI in its JFIF header, in the two density fields, with the unit set to dots per inch. Software that reads those fields can print the image at the page's real size.
Choosing a DPI for the job
| DPI | Use it for |
|---|---|
| 72 | The smallest files and on-screen previews |
| 150 | The default: sharing, and most upload forms |
| 200 | Small print that must stay legible |
| 300 | Printing |
Here is how to handle each of three kinds of rule a form may set.
A pixel size. Work out the DPI it needs with DPI = pixels × 72 ÷ points, then take the next setting up. Say a form wants at least 2,000 pixels on the long side of an A4 page: 2,000 × 72 ÷ 841.89 = 171.04 DPI, just over 171. There is no such setting, so choose 200, which gives 2,339 pixels.
A DPI. Choose it directly if it is one of the four settings, and otherwise take the next one up. The DPI is written into every JPG; only a page too large to draw at that DPI gets a lower one (see below).
A file size in KB. Start at 150 DPI and lower the quality slider, and drop to 72 DPI if the file is still too large. For an exact KB limit on one image, finish on the Image Resizer in KB.
Pick the lowest setting that meets the stated rule, because every step up costs file size. Whether a portal accepts the result is decided by its own rules; the converter only makes the file to your settings. If the form wants the PDF itself at a minimum file size, Increase PDF Size pads it up to a target size, and Text to PDF makes a PDF from typed text.
What a higher DPI costs
Pixels grow with the square of the DPI:
pixels at 300 DPI ÷ pixels at 150 DPI = (300 ÷ 150)² = 4
An A4 page is about 2.17 million pixels at 150 DPI and 8.70 million at 300. File size grew less than that on the converter's test page with a photo: the JPG was 87 KB at 150 DPI and 240 KB at 300 DPI, about 2.8 times larger. Plain text pages were about 10 KB at 72 DPI and 30 KB at 150 DPI. These are measurements of particular pages; yours will differ.
Memory is the other cost. A canvas holds 4 bytes per pixel while a page is drawn, so an A4 page at 300 DPI needs a canvas of about 35 MB. The converter draws one page at a time and releases each canvas before the next, so only one page's canvas is held at once. In tests, 5 pages at 300 DPI took about 1.0 second on a desktop and 1.9 seconds with the processor slowed fourfold to imitate a phone.
JPEG quality
The quality slider runs from 50% to 100% in steps of 5 and starts at 90%. DPI decides how many pixels there are; quality decides how much detail JPEG compression keeps in them. The converter hands the setting to the browser's JPEG encoder as a fraction, so 90% goes in as 0.9.
At 90%, text stays crisp. Lowering the slider trades sharpness for a smaller file, and on a document small print is where to check for the loss. When a form sets both a pixel size and a KB limit, keep the DPI that meets the pixel rule, lower the quality until the size fits, and zoom in on the smallest print before you send it.
Why a large page comes out at a lower DPI
iPhone browsers refuse a canvas above 16,777,216 pixels, which is 4,096 × 4,096, and draw it blank without an error. The converter also keeps each side at or below 16,384 pixels. When a page at your chosen DPI would break either limit, it is drawn at the highest whole-number DPI that fits. The starting point is the lower of these two, rounded down:
DPI = 72 × √(16,777,216 ÷ (width in points × height in points))
DPI = 16,384 × 72 ÷ longest side in points
Take an A0 poster, 2,384 × 3,370 points, asked for at 300 DPI. That would be 9,933 × 14,042 pixels, about 139.5 million. The first formula gives 72 × √(16,777,216 ÷ 8,034,080) = 104.05, so the page is drawn at 104 DPI: 3,444 × 4,868 = 16,765,392 pixels, just under. At 105 DPI it would be 17,089,455, just over.
A till receipt 80 mm wide and 3 m long (226.77 × 8,503.9 points) hits the side limit instead: 16,384 × 72 ÷ 8,503.9 = 138.7, so it is drawn at 138 DPI, 435 × 16,299 pixels.
At 300 DPI the pixel limit allows a page about 1.93 times the area of A4, so it is not only posters that are affected: an A3 sheet at 300 DPI is capped too.
The DPI is rounded down to a whole number so that the value written into the JPG is true, and a note with the results says when pages were capped. A capped page still prints at its real size: 3,444 ÷ 104 = 33.1 inches, the same as the poster's 2,384 ÷ 72 = 33.1 inches, just with fewer pixels per inch.
Password-protected PDFs
When a PDF needs a password to open, the converter asks for it and hands it to pdf.js, the PDF engine built into Firefox, which draws the pages. A wrong password gets a message that it did not open; passwords are case-sensitive. The password is used in the tab to open the file and is not stored or sent anywhere.
There is no way round a password you do not know. The protection also stays behind in the PDF. The JPGs are ordinary images with no password, so anyone who receives them can open them. Share them with the same care as the original.
Many pages: ranges, runs and the ZIP
PDFs up to 100 MB open whatever their page count, and up to 500 pages convert in one run. Leave the page box empty for every page, or type pages and ranges:
| You type | Pages converted from a 20-page PDF |
|---|---|
| 1-3,5,8- | 1, 2, 3, 5 and 8 to 20 |
| 15-40 | 15 to 20; the range stops at the last page |
| 25 | None; the message says the PDF has 20 pages |
| 9-4 | None; the message suggests 4-9 |
A 1,200-page PDF takes three runs: 1-500, 501-1000 and 1001-, the last making 200 JPGs. File names are padded to the width of the page count, so for a file called report.pdf the pages run from report-page-0001.jpg to report-page-1200.jpg and sort in page order once the runs share a folder.
Each page has its own Download button, and Download all puts the run's JPGs in one ZIP named after the PDF, such as report-jpg.zip. Starting a new run clears the JPGs of the last one, so download each run's ZIP before you convert again. Every run's ZIP is given the same name, so rename each one as it arrives to keep track of which pages it holds.
The ZIP library stores files without compressing them, so the ZIP is about the sum of the images, and the Download all button shows that total. If 30 pages were each like the test page with a photo, the ZIP would come to roughly 30 × 87 = 2,610 KB at 150 DPI and 30 × 240 = 7,200 KB at 300 DPI.
The finished JPGs stay in the tab's memory until you leave the page, choose another file or start another run.
Fonts and scanned images
Fonts a PDF embeds are drawn exactly; a font it only names is drawn with your device's own fonts, so spacing can differ slightly from Adobe Acrobat, and a phone and a laptop can give slightly different results. Check tables and tightly laid-out forms before you send them.
A scanned page may store its image as CCITT fax, JBIG2 or JPEG 2000. pdf.js decodes these with helper modules it loads from this site the first time it needs one. If a module cannot load, the page is still drawn but that image is left out, which on a scanned page can leave little or nothing to see. Look at every thumbnail before you send the files.
What this tool assumes
- The PDF is drawn in your browser tab with pdf.js and never uploaded. The converter's only requests are for pdf.js and its helper files, from this site.
- Pages are drawn one at a time on a white background, at 72, 150, 200 or 300 DPI, and rounded to whole pixels.
- A page over 16,777,216 pixels, or over 16,384 pixels on a side, is drawn at the highest whole-number DPI that fits.
- Files up to 100 MB and up to 500 pages per run.
- Browsers without the newest JavaScript features, such as older iOS Safari and old Android WebViews, get pdf.js's legacy build, which runs on the page itself: slower, and the page is busier while it draws.
- Whether a portal or person accepts the images is up to them.