Changing the page size of an existing PDF is possible, but it works differently depending on what you actually need. There's an important distinction between scaling the content to fit a new page size and changing the page dimensions while leaving the content untouched โ both are useful in different situations, and they produce very different results.
The Print-to-PDF Method: Simplest for Rescaling
If you want to change an A4 document to Letter size (or vice versa) and have the content scale to fit, the print-to-PDF approach works well. Open the PDF in any viewer, go to Print, and set the paper size to your target dimensions. Enable "Fit to Page" or "Scale to Fit" in the print settings, then print to a PDF printer (Microsoft Print to PDF on Windows, Save as PDF on Mac). The result is a new PDF at the specified page size with all content scaled proportionally.
For A4 to Letter conversion specifically, the size difference is small enough (A4 is slightly taller and narrower) that scaling often has no visible effect at normal reading zoom. For more dramatic size changes โ say, converting a standard document to A3 โ the scaling becomes obvious and text will appear larger than intended.
Changing Page Dimensions Without Scaling Content
Sometimes you want to change the page canvas size without touching the content โ for example, adding white space around a document for binding, or trimming excess margin. This is a crop or resize operation rather than a scale operation, and it requires a dedicated PDF tool.
In Adobe Acrobat Pro, the Page Boxes tool under Tools โ Edit PDF lets you set the MediaBox, CropBox, and other page dimensions precisely. WukongPDF's PDF Editor includes a crop tool that lets you drag to define the visible page area, effectively trimming the page size without affecting the content within. For adding white space rather than trimming, the print method with a larger paper size and no scaling places the original content within the larger page at its original size.
Cropping vs. Resizing: A Common Point of Confusion
Most free PDF tools that claim to "resize" pages are actually cropping โ they change what part of the page is visible, not the underlying page dimensions. In PDF terms, adjusting the CropBox hides content outside the defined area but doesn't delete it. The content is still in the file; it's just not shown. This is fine for most purposes but means the file size doesn't decrease proportionally, and some tools may reveal the cropped content if they ignore the CropBox.
True resizing that actually removes content outside the new boundaries requires modifying the MediaBox and clipping the content streams โ a more destructive operation that's harder to reverse. For archival documents, cropping (preserving hidden content) is generally safer than true resizing.
When to Go Back to the Source Instead
If you created the document yourself and need it in a different page size for a specific purpose โ a print job, a submission that requires Letter instead of A4, a booklet layout โ the cleanest approach is to change the page size in the source document and re-export. In Word: Layout โ Size. In Google Docs: File โ Page Setup. Changing it at source ensures the layout reflows correctly for the new dimensions rather than just scaling whatever existed before.
Post-processing an existing PDF to change page size is the right call when you don't have access to the source file or when the document has complex layout that you don't want to rebuild.