When you need to merge multiple files into one PDF, you may think of Adobe Acrobat which is the most popular PDF editor and converter. With Acrobat DC, you can combine two or more files, images, documents, spreadsheets, emails, PDF, and other file types, and arrange them in one compact PDF. For example, you can refer to this tutorial to merge multiple images to PDF and these steps to combine two PDF files using this PDF tool on your computer. One issue of merging several files together using Acrobat you may encounter is that it just takes what files you have without modifying the page size. For example, if your source PDF file size is 8.5×11 in, the merged PDF will have the same page size; if you have included PDF files or images with different sizes, some PDF pages may be bigger or smaller than others in the output PDF file. Today, we’ll discuss how to keep pages the same size when assembling a PDF of multiple pictures, images, and/or PDF documents.
Make merged PDF pages the same size
Adobe Acrobat has a virtual printer called Adobe PDF Printer which you can use to create PDF files. After you have combined the PDF and/or image files in Acrobat. Instead of saving it to your computer as a new PDF file, go to its File menu, select Print.
In the Print dialog, choose Adobe PDF in the Printer drop-down menu, then set your sheet size, orientation as if you are using a printer. By default the paper size will be A4, you can change it from Page Setup…. However the “Choose paper source by PDF page size” is enabled by default. We can disable this option so Acrobat won’t always just take your original documents with all their page sizes. Select Fit, not Actual size, so Acrobat will scale your PDF pages to the selected paper size. If there are images with different resolutions you want to merge together, Acrobat can then scale the images and print the entire images without cropping any part of them. Disable the auto rotate option (Auto portrait/landscape), choose either Portrait or Landscape if you have included both portrait and landscape images in the same PDF file. The orientation setting may add white bars at the top/bottom or left/right of some pages when you need to print mixed page sizes. Preview the PDF file at the lower right section to ensure all pages have the same size. When you are ready, click the Print button at the bottom right corner. You will see the Save PDF File As dialog where you can select a location and give the new file a name. Finally click the Save button to create a new document that contains all selected PDF pages with exactly the same size. You can open the resulting PDF file in whatever PDF app you have and your new PDF will be uniform in page sizes.
Adjust the page size of existing PDF file
You can also use this printing a PDF to a PDF method to import an existing PDF file with varying page sizes created with Adobe Acrobat or other PDF program into the Acrobat and use the Adobe Print utility to adjust the page size to the same and export as a new PDF again.
The merits and demerits
The print to PDF method will reduce PDF file size greatly. Meanwhile it will lower the quality of your PDF file and remove all interactive elements, flatten layers, convert certain colors, etc.
Scale PDF pages using Preflight tool in Acrobat
Adobe Acrobat Pro has the Preflight tool that you can use to scale PDF files on computer without losing those interactive elements as mentioned above.
Update Apr 2019: removed amzn.