When merging a very large number of PDF files using the PDF Merge API (/v1/pdf/merge), you may encounter a timeout error — even when using asynchronous mode.
This behavior is expected due to processing time limits and how serverless infrastructure handles long-running tasks.
Why Timeouts Occur
PDF.co uses Lambda-based compute for processing. This means:
Synchronous requests have a strict limit of 30 seconds
Asynchronous requests have a maximum processing time of 3 minutes
If your merge operation requires more than these limits — which often happens when merging hundreds of PDFs — the job may:
Time out
Fail with a processing error
This is especially common when merging large files in a single request.
Recommended Workaround: Merge in Batches
To reliably handle large merges, we strongly recommend splitting the operation into smaller batches.
Suggested batching strategy:
Group PDFs into batches of ~150–200 files each
Merge each batch separately using
/v1/pdf/mergeTake the few resulting merged PDFs (e.g., 3 or 4 final files)
Run one last merge to combine those batch outputs into the final single PDF
Why This Works
Batch merging:
Keeps each merge task well below the time limits
Prevents long-running operations from timing out
Reduces server load and speeds up processing
Offers much more predictable and stable performance
Works reliably even for workflows requiring 500–1,000 PDFs
Example Workflow
Batch 1: merge PDFs 1–200 → result batch1.pdf
Batch 2: merge PDFs 201–400 → result batch2.pdf
Batch 3: merge PDFs 401–542 → result batch3.pdf
Final merge: batch1.pdf + batch2.pdf + batch3.pdf → final_output.pdf
Need Further Help?
If you're unsure how to structure your batching logic, or if you need help optimizing your workflow in Make, Zapier, n8n, or custom code — feel free to reach out and we’ll be glad to assist.