Resize PDF Free Online — Change PDF Page Size to A4, Letter & More

📐Resize PDF Online — Why It Comes Up More Often Than You’d Expect

Resize PDF pages online in seconds — it sounds like a niche operation, but I’ve needed it probably a dozen times in the last year alone. The most recent one was a supply chain audit report a client sent over. The whole document was formatted for US Letter, but everyone in our European office was printing on A4. Every page was printing with a clipped right edge and weird white gaps at the bottom. It took me about 45 seconds to resize PDF pages from Letter to A4, and the problem just went away. Nobody had to reformat anything in Word.

The other situation that comes up constantly is academic papers. Most papers from US journals come as Letter PDFs. If you’re printing and binding them for study, they look off on A4 paper. Just resize PDF to A4, scale content to fit, and they print correctly with normal-looking margins. I’ve done this probably 30 times for students and researchers.

This tool handles the whole operation in your browser using pdf-lib, an open-source PDF manipulation library that runs 100% client-side. Nothing goes through any server. The resize operation rewrites each page’s MediaBox to the new dimensions and — if you’ve chosen to scale content — applies an affine transformation to fit the existing content proportionally into the new page area.

📋How to Resize PDF Pages — Step by Step

The whole thing takes under a minute. Here’s the process:

👥Real Situations Where You Need to Resize PDF Pages

Here’s when I actually reach for this tool — plus the requests I get from people who’ve figured out it solves their specific problem:

• US Letter PDF printing on A4 paper (and vice versa): This is the big one. The difference between Letter (215.9 × 279.4 mm) and A4 (210 × 297 mm) is subtle but it causes real clipping and margin problems when printing across regions. Resize PDF from one to the other and the problem disappears. I had a 60-page proposal that was Letter format — resizing it to A4 before sending to a European client took about 8 seconds total.
• Making a full-size A4 document into A5 for a booklet: If you need to print a document as a folded A5 booklet, you first need the content at A5 size. Resize PDF from A4 to A5 with content scaling on, and each page shrinks proportionally. Then print two-per-sheet in your print dialog for a clean booklet format.
• Fixing oversized A3 scans for standard printing: Scanners sometimes output at A3 even when the original document was A4 — especially older flatbeds. Resize PDF pages from A3 down to A4, scale content to fit, and you’re back to normal. The output looks identical to a direct A4 scan.
• Preparing PDFs for e-readers: Kindle and other e-readers have their own aspect ratios. A standard A4 document has a lot of wasted space when read on a Kindle Paperwhite. Resize PDF to something closer to 90 × 117 mm (close to Kindle’s screen) and the text fills the screen properly. It’s not a perfect solution but it’s dramatically better than the alternative.
• Legal documents in the wrong paper size: US Legal size (215.9 × 355.6 mm) is common in contracts and court filings. If you receive a Legal PDF and need to print it on Letter or A4, the content just gets cut off unless you resize first. Resize PDF to your printer’s paper size and then everything fits.
• Technical drawings exported at the wrong size: CAD software sometimes exports at A1 or A0. If you only need to share a reference copy at A4 size, resize PDF down to A4 with content scaling — it’ll be smaller but fully readable.

🔬How Resize PDF Actually Works — The Technical Side

When you resize PDF pages, there are two separate things happening. Understanding both helps you get the result you actually want.

• Changing the MediaBox: The MediaBox is the PDF’s definition of the physical page size — it’s what tells a PDF viewer “this page is 595 × 842 points” (that’s A4). When we resize PDF pages, we write a new MediaBox value for every page. Without content scaling, this just changes the canvas size — content stays exactly where it was and may appear too small or get cut off if the new size is smaller.
• Scaling content with an affine transform: When “Scale content to fit” is enabled, we calculate the scale factor needed to fit the original content into the new page dimensions, then apply a PDF content transform matrix. This is mathematically lossless for vector content (text, shapes, lines) — quality is identical. For embedded images it’s still high quality because we’re not resampling pixels, just changing how the PDF renderer positions them.
• Aspect ratio differences: A4 and Letter have slightly different aspect ratios (A4 is taller relative to its width). If you resize PDF from Letter to A4 with content scaling enabled, the content will be scaled to fit the width and there’ll be a small amount of white space at the bottom. This is expected — it’s the same behavior you see when you print a Letter document on A4 paper with “fit to page” enabled.

The PDF spec defines page dimensions in points (1 pt = 1/72 inch). This tool handles the mm-to-points conversion internally, so the numbers you see in mm are accurate to within 0.01mm.

📏PDF Page Size Reference — A4, Letter, Legal and More

Here’s a quick reference for the most common PDF page sizes, in both mm and points (the unit PDFs use internally). This comes up when you’re trying to match a specific paper size for printing or client requirements.

If your target size isn’t in this table, use the Custom option and enter the exact millimeter values you need. The tool converts to points internally.

💡Tips for Getting the Best Results When You Resize PDF

• Always enable “Scale content to fit” for cross-region conversions: If you’re converting Letter → A4 or vice versa without scaling, the content will stay at its original size on the new page and look off-center with uneven margins. Turn scaling on and everything repositions correctly. The only time to turn it off is if you deliberately want to keep content at its original absolute size on a different canvas.
• Check the orientation before you resize PDF: I’ve made this mistake — resizing a landscape PDF to A4 portrait without noticing. The content ends up squished or with black bars. Check the live preview and switch to landscape orientation if your document is wider than tall.
• For A5 booklets, resize then print 2-up: Resize PDF to A5 with content scaling on. Then in your printer dialog (or using a print-to-PDF driver), select “2 pages per sheet.” You get a properly proportioned A5 booklet printed on A4 paper, with correct margins all around.
• Custom size for e-readers: Amazon Kindle Paperwhite has a screen ratio of roughly 3:4. Try 90 × 120 mm as a custom size when preparing a PDF for Kindle sideloading. The text will fill the screen much better than an untouched A4 PDF.
• Resize PDF before adding watermarks or signatures: If you need to add a watermark or digital signature to a document, resize it to the correct page size first. Repositioning overlays after resizing is more work. I learned this one the hard way working on client contracts.
• After resizing, use Compress PDF to trim file size: Resizing itself doesn’t increase file size meaningfully, but it’s a good moment to also run Compress PDF if the file is large. The two operations together take under a minute total.

❓Frequently Asked Questions

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