How to Merge PDFs with Different Page Sizes

Introduction

Handle A4/Letter/Legal mismatches when merging, with normalization strategies. This guide assumes you have a basic understanding of PDF files and builds on that knowledge with more detailed techniques and real-world scenarios. Whether you're using PDFly's free online tools or building your own workflow, this guide covers everything you need to know about how to merge pdfs with different page sizes.

Understanding Merging

Merging PDFs combines multiple documents into a single file, streamlining document management and sharing. Whether you're compiling reports, combining scanned pages, or assembling a portfolio, PDF merging is one of the most common document operations. Modern browser-based tools like PDFly let you merge PDFs instantly without uploading files to a server.

Key Concepts You Need to Know

Merging combines all pages from multiple PDFs into a single document
Page order is determined by the sequence in which files are added
Bookmarks and table of contents can be preserved or recreated during merging
Form fields with duplicate names may conflict when merging — renaming is essential
Merging is non-destructive — original files remain unchanged

1

Prepare Your Files

Gather all PDF files you want to merge. Ensure they're accessible and note the desired order.

2

Upload or Select Files

Open PDFly's Merge PDF tool and drag your files into the upload area, or click to browse.

3

Arrange the Order

Drag and drop the file thumbnails to set the correct page sequence for your merged document.

4

Merge and Download

Click the merge button and download your combined PDF. The process runs entirely in your browser.

Tips & Best Practices

Always verify page order before merging — rearranging after merging is harder

Use consistent page sizes across source files for a professional result

Consider adding a cover page or table of contents to the merged document

Flatten form fields before merging to avoid field name conflicts

Merge in small batches if working with very large numbers of files

Common Mistakes to Avoid

Avoid These Common Pitfalls

Merging PDFs with different page sizes creates inconsistent documents
Form fields with duplicate names merge into a single field — always rename first
Merging encrypted PDFs requires passwords and may fail silently
Large merges can consume significant memory — process in batches
Bookmarks from source files may conflict or create duplicate entries

Quality Checklist

0 / 5 completed

All source PDFs are accessible and in the correct order Page sizes are consistent across all source files Form fields have been flattened or renamed to avoid conflicts Bookmarks have been planned for the merged document The merged file has been tested and verified

Key Takeaways

PDF merging is lossless and non-destructive

Always verify page order and flatten form fields before merging

Browser-based merging is free, private, and requires no installation

Frequently Asked Questions

Yes! PDFly's Merge PDF tool is completely free, requires no signup, and processes files entirely in your browser. Your documents never leave your device.

You can merge as many PDFs as your browser's memory allows. For very large numbers of files (100+), consider merging in batches for better performance.

No, merging is a lossless operation. The quality of images, text, and formatting in your original PDFs is preserved exactly in the merged document.

Yes, but you'll need to provide the password for each protected file. The merged document will not be password-protected unless you apply security separately.

How to Merge PDFs with Different Page Sizes

Introduction

Understanding Merging

Key Concepts You Need to Know