Now, it appears that it is also the combination of file names and that this causes the problem. I have created a separate page with the previous answers. Please consider donating to the author if you use Adobe Reader. How to Combine PDFs in Windows 8 Using the new File Explorer, and opening the folders with files you wish to combine with each other, you will need to make sure the file names on the folders and the combined file names do not have the same file extension. For example, “My Documents.docx” and “My Documents.pdf” will not merge. Do the following; 1. On the File Explorer of your computer, go to My Documents and select File > New. 2. A new folder is created with the name My Documents. 3. Copy both of the files we intend on combining into that folder. 4. In that folder, go to My Documents > My Documents.docx and My Documents > My Documents.PPTX, and copy the files into that folder. 5. Finally, open My Documents > My Documents.pdf in another application, and you are all set to go. Please note that the files in the folder I have provided, that have different extension than any of the files in the other folders, are not in my example shown above, because the extension of that folder is not .docx. If a file is not in .pdf format when installed in the Documents folder, please install the application using an alternative file extension like .jpg or .png. There will be some files that are not present in my example shown above but will be fine to combine since you have copied all the files from My Documents. Pdf to My Documents for example. How to Merge Multiple PDFs into One File in macOS To combine multiple PDFs into a single file. 1. Open TextEd it. 2. Press Command-V (paste) at the top of the screen. 3. Paste the following into TextEd it as is. If text is entered, it won't work as the new text file is going to be larger than 2 GB. But, the file size is only an indicator for this.