The internet is full of online tools that want to help you merge PDFs. For free, no less. It sounds like a good deal, but it’s hard to know for sure. Like many files, PDFs contain metadata that could identify you, and if you choose poorly, your uploads might end up in the wrong hands.

Of course, not all of these programs could potentially ship your tax documents off to identity thieves. Some are run by well-regarded companies, and others may have strong security protocols. The thing is, you just want to join some files together, not spend precious time researching the privacy practices of a virtual stapler. And if you’re working without an internet connection, even the most trustworthy web-based tool can’t help you.

Thankfully, Apple has a PDF combiner built right into macOS, and while Windows doesn’t have anything similar, there are some workarounds you can try.

How to combine PDFs on macOS

There are two ways to merge PDFs on Apple’s operating system, though one allows for more customization than the other. It’s also worth noting that these techniques allow you to combine any type of image file into a PDF, so you can tack a JPEG of a receipt onto the end of a PDF invoice if you want.

To slam two or more files together, end-to-end, nothing more, open Finder and make sure it’s in Gallery view. To be sure, click on View or the view icon at the top of the Finder window (the only one with both up and down arrows) and choose as Gallery. Then, select your files in the order you want them to appear in the PDF. Command + click will probably be your best selection method, but other approaches (like Shift + click to select consecutive files and, if you’re feeling wild, Command + A to select all) will work too. If you’re having trouble identifying the files you want, create a new folder just for them and work from there. Finally, click Create PDF from the options in the bottom right of the Finder window to save your creation to whatever folder you’re working out of.

[Related: 38 advanced Mac keyboard shortcuts for productivity]

For more freedom and organization options, open one PDF in Preview. To merge it with another one, make sure page thumbnails are visible in the Preview window (go to View > Thumbnails if they aren’t). To add an entire new PDF after a specific page, select that page’s thumbnail, go to Edit, Insert, and then Page from File. Choose the PDF you want to add, and click Open. Finish by clicking File, Export as PDF, giving your new PDF a name, and hitting Save.

If you really need to work one page at a time, open both PDFs in separate Preview windows. Then, simply drag one or more page thumbnails from one into the thumbnail sidebar next to the other. This will copy and insert the selected pages, and you can continue to move and delete the pages from the thumbnail bar as you please. You can also edit and rearrange a single PDF this way.

How to combine PDFs on Windows

You’ll need to work to merge PDFs on Windows machines. There’s no built-in capability like there is on macOS, so you’ll have to get creative. That likely means you’ll have to download a separate program to get the job done.

Before we talk about downloading programs, though, we’ll mention that if all you want to do is combine a bunch of images into one PDF, you can paste those images into Microsoft Word or any other program that can display images and offers a Print to PDF option. You can also convert a PDF into image files with an image editor and do the same thing. This is, admittedly, a low-tech choice and it’s probably not going to look great. But if you’re saving something for personal use, you may not care.

[Related: How to convert any file to any format]

If you have an internet connection, your best bet is to download a third-party program, and we like open-source tool PDFsam for the job. If you do too, make sure you select PDFsam Basic from the program’s website and choose Windows from the download options (it also works on macOS, Linux, and through a command line interface). Once it’s installed, your options include merging, splitting, and rotating PDFs. You can also purchase a premium version that includes features like PDF creation and page deletion at prices starting at $69 a year. But if you’re just trying to stick one class assignment to another, you probably don’t need to spend all that cash.

To merge PDFs end-to-end, click Merge, then drag and drop the files into the workspace. You can also click Add and select files that way. From there, you can drag them up and down the list to rearrange them. You can’t use PDFsam Basic to mix and match pages, but you can use the Split function to break a multi-page PDF into smaller chunks (down to individual pages if necessary). Then use Merge, import all those building blocks, and stitch them into a new PDF.

As a last resort, you can use image-editing software like open-source GIMP to merge PDFs, but it involves a bit of work. The only benefit to this strategy is that you may already have such a program on your computer (given the option, we’d prefer downloading PDFsam).

Using GIMP as an example, open one PDF and make sure the dropdown menu next to Open pages as says Layers. Next, go to File and use Open as Layers to open the PDFs you want to merge. All pages will be stacked on top of each other, so you’ll have to click on Image, Canvas Size, and increase the height of your canvas to fit all the pages. Then use the Move tool (a plus sign made of arrows) to drag each page where it needs to be. Finally, crop your file to eliminate any dead space and use File > Export as to save it as a new PDF. That’s… a lot, so we really only recommend this if you’re in a tight spot, can’t download a new program, and aren’t merging absolutely massive PDFs. The manual rearrangement can be truly tedious.

And if you really must use a free online service, please, at least delete your metadata first.