Why does merging make PDFs larger?
Here is a counter-intuitive problem: you merge two 1MB PDF files, expecting a 2MB result, but instead get 5.8MB or even larger. This happens because each source PDF's fonts, images, and metadata are fully preserved, and many tools duplicate the same data multiple times during the merge process.
I ran into this myself while working on a project proposal. I had five PDFs — each around 2-3MB, totaling about 12MB. After merging, the result was a whopping 35MB. I was shocked. It turned out one of the PDFs contained high-resolution screenshots at 3000x2000 pixels, eating up several MB each. Another had an entire Chinese font family embedded, adding 8MB just for the fonts.
After separately compressing those two files — reducing screenshot resolution to 150dpi and switching fonts to subset embedding — the merged result was just 15MB. That is less than half the original merged size. Now I always check each PDF's properties before merging to spot any "fat" files first.
Solution 1: Compress each PDF before merging
The most effective approach is to compress each PDF individually before merging. Images are the main culprit — compressing them can shrink each source file by 50% to 80%. You can use our Image Compressor or any PDF compression tool to handle this.
Here is a quick checklist: check each PDF for large images (over 2000px), embedded fonts, and scanned pages. If any of these apply, compress that PDF first. A minute of preprocessing can save you 50% or more on the final file size.
Solution 2: Remove duplicate embedded fonts
Merge tools often embed the same font multiple times — once from each source file. If three source files all use "Arial," the merged PDF contains three complete copies of Arial.
If you have Adobe Acrobat Pro, use the "Preflight" tool to detect and remove duplicate fonts. For a free option, open the PDF in LibreOffice Draw and re-export it — this process automatically deduplicates fonts. I tested this on a 4-file merged PDF, and LibreOffice reduced it from 22MB to 14MB.
Solution 3: Compress the merged result again
This might seem redundant, but it works. A colleague of mine always compresses the final merged PDF as a finishing step. He says it is the most reliable method because it doesn't require investigating each source file individually.
One caveat: a second compression pass may reduce image quality. If your PDF contains high-resolution images intended for print, keep the quality above 80% during the second pass.
Solution 4: Clean up metadata
Each PDF carries metadata — title, author, subject, keywords. During merging, these get accumulated. The total is usually small, but every kilobyte counts.
In Adobe Acrobat, go to File > Properties > Description and remove unnecessary information. Many online PDF tools also have a "clean metadata" feature. This step typically saves a few hundred KB to a few MB — not huge, but it costs nothing and takes 10 seconds.
Solution 5: Choose the right tool
Not all merge tools are created equal. Some do a better job at optimizing the output. Our PDF Merger is designed to minimize bloat by avoiding redundant data inclusion.
Summary
Post-merge file bloat is normal but preventable. Compress first, merge second, clean up metadata, and pick the right tool. These three steps solve most file size issues.