Additional information is provided for combining PDFs on mobile devices and using Combine PDF and Merge PDF.
How to Merge PDF Files With Adobe Acrobat DC
Adobe Acrobat Reader is free.
To further manipulate these files or combine multiple PDFs into one, installAcrobat DC.

Open Acrobat DC and selectTools>Combine Files.
SelectAdd Filesto add as many files as you would like.
SelectAdd Open Filesto include documents you’re currently viewing.

Adjust the order, including that of individual pages, by dragging and dropping each to the desired location.
SelectCombineto complete the process.
Open one of the PDF files you want to combine in the Preview app.

Go to theViewmenu and selectThumbnails, if it isn’t selected, to turn on the thumbnail preview.
To toggle the Thumbnails view, pressOption+Command+2on the keyboard.
Inserted PDF pages appear after the selected page.

SelectEdit>Insert>Page from File.
In aFinderwindow, locate the second PDF file you want to import into the current one, then selectOpen.
Repeat steps 4 and 5 for each additional PDF file you want to import.

Drag thumbnails to change the order of pages.
When you are satisfied with the combined PDF, selectFile>Save.
How to Combine PDF Files With PDF Merge
Several websites offer PDF merging services.

Many are ad-driven and free of charge.
There is a limit of 10 MB for files that are uploaded.
PDF Merge also offers an affordable downloadable app that merges PDFs offline.

Only a Windows version is available.
Go to thePDF Mergewebsite.
Choose the first PDF you want to merge, then selectOpen.

SelectChoose Fileagain for the other files you want to combine.
If you run out of slots, selectMore Files.
SelectMergeto combine the selected files.

The files are combined in the order in which you select and upload each file.
Download the final combined PDF to your system.
Merge up to 20 files, including images, into a single PDF file for free.

Combine PDF claims to delete all files from their servers within one hour of upload.
Go to theCombine PDFtab, if it isn’t selected.
Drag the PDF files to the PDF Merge web page.

Or, upload the files in the traditional fashion by selectingUpload Files.
Adjust the order of the uploaded files by dragging the files in the queue.
The merged PDF file,combinepdf.pdf, downloads to your system.

Select eitherFrom Google DriveorFrom Dropboxto open a login window.
type your login credentials for the cloud service to see your cloud-based files.
choose the files you want to upload, then selectChoose.

These options are the most reliable: