Merging
Fix “PDF Files Not Merging” Error Easily
A troubleshooting playbook for anyone who keeps seeing “cannot merge PDF” pop-ups in Acrobat, Preview, or online tools.
Table of contents
Identify the root cause
Merging fails for a handful of repeatable reasons: password protection, file corruption, incompatible PDF versions, or extremely large assets. Rather than guessing, scan the error message closely. Acrobat might say “The file is encrypted,” while MyPDFHero will politely suggest removing a password.
Once you know the category, the fix usually takes less than two minutes.
Try the tools mentioned in this guide
Every workflow here is powered by MyPDFHero. Jump straight into the tool that fits your task.
Compress PDF
Shrink assignments, contracts, or reports to fit email and LMS limits.
Open tool →Merge PDF
Combine chapters, receipts, or scans before submitting a single file.
Open tool →JPG to PDF
Turn photos, scans, or screenshots into polished PDFs in seconds.
Open tool →Remove passwords or permissions
Locked PDFs cannot merge because the tool cannot read their content. Open the PDF in your viewer, enter the password, then re-save it without protection. On macOS, choose File → Export as PDF and leave the password fields blank. On Windows, print the file to PDF to flatten it.
After unlocking, try the merge again. MyPDFHero handles the rest automatically.
Repair corrupted PDFs
If a PDF refuses to open or merge, upload it to Google Drive and open it there. Drive often repairs mild corruption. Alternatively, export each page as an image and rebuild the PDF using the JPG to PDF tool. This gives you a clean, merge-ready file.
Corruption typically happens when uploads are interrupted or when third-party fax apps generate malformed PDFs.
Normalize page sizes and orientations
Some tools crash when mixing landscape slide decks with portrait scans. Use Preview (Mac) or the browser-based MyPDFHero converter to rotate pages before merging. Keeping consistent page sizes also reduces layout surprises downstream.
If the PDF contains huge A3 or poster-sized pages, resize them to Letter/A4 equivalents and try again.
Check for unsupported content
Embedded video, 3D objects, or excessive form scripting can block merges. Flatten the PDF by printing to PDF or exporting to a simpler version. Then rerun the merge.
Remember that recruiters and admissions officers rarely need multimedia, so stripping it out not only fixes errors but improves compatibility.
Step-by-step workflow
Follow these practical steps inside MyPDFHero or your operating system to complete the task quickly.
Step 1
Read the error message
Note whether it mentions encryption, corruption, or unsupported features.
Step 2
Unlock or repair the file
Remove passwords or rebuild the PDF from fresh exports.
Step 3
Standardize formatting
Rotate, resize, or flatten pages to match the rest of the batch.
Step 4
Merge again in MyPDFHero
Upload the cleaned PDFs and confirm they combine without errors.
Step 5
Archive the fixed files
Store the corrected PDFs so you never face the same error twice.
Official resources
Validate your workflow with trusted documentation from Google, Microsoft, Adobe, and other official sources.
- Adobe: Remove security
Official steps to unlock PDFs before merging.
- Google Drive PDF viewer
Drive can repair mildly corrupted files during upload.
- Microsoft Print to PDF guide
Use print-to-PDF as a way to flatten troublesome files.
Frequently asked questions
Why does Acrobat say my PDFs cannot be merged?
They may be password-protected, damaged, or contain multimedia that Acrobat cannot concatenate. Unlock or flatten them first.
How do I know if a PDF is corrupted?
If it will not open in multiple viewers or throws errors during merging, assume corruption and rebuild it from the source or images.
Can different page sizes break a merge?
Some tools struggle with drastically different page sizes. Normalize or rotate pages to maintain consistency.
What if the merge stops midway?
Check your connection and file sizes. Slow networks can interrupt uploads, causing incomplete merges. Re-upload on a stable connection.
Is there a file size limit when merging?
Yes, every tool has limits. MyPDFHero supports up to 20 MB per file. Compress the PDFs first if you hit a ceiling.
Can I get help from support?
Absolutely. Email hello@mypdfhero.com with the problematic files (if policy allows) and we’ll inspect the metadata.
Related reading
Expand your PDF toolkit with more long-tail guides from MyPDFHero.
How to Combine PDF Files on Windows and Mac
We compare Preview, Microsoft Print, and the MyPDFHero Merge tool so you can consolidate PDFs whether you are on Windows, macOS, or Chromebook.
Read more →How to Merge PDFs on Mobile (Android + iPhone)
Step-by-step instructions for Android, iOS, and ChromeOS users who need to stitch PDFs without touching a laptop.
Read more →PDF Too Large to Upload? Here’s How to Fix It
Covers emergency fixes for admissions, visas, payroll, or LMS portals that refuse a PDF because it exceeds their threshold.
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