The Preview app ought to have a better opinion of itself. This stock Mac app goes around calling itself "Preview" when it can do so much more than just preview files. To help Preview form a more positive view of itself and help you get the most out of the app, I present 9 things Preview can do beyond showing you previews of files.
1. Batch changes
Preview can open more than one file at a time. If you highlight multiple files in Finder or your desktop, you can open them all in one Preview window. Thumbnails of the files are shown in the sidebar along the left edge of the window.
If you click to select one of the thumbnails and then hit Command-A to select all, then you can make changes to all of the files. You can, for example, resize images by going to Tools > Adjust Size. Or you can change the file format -- PNG to JPEG, for instance -- by highlighting the images and going to File > Export Selected Images. On the export window where you choose the destination for your export, hit the Options button to choose the file format.
2. Easy annotation tools
Need to markup a screenshot to explain or highlight something? Go to Tools > Annotate or click the little toolbox button at the top of Preview and you can add arrows, ovals, rectangles, text and more to a file. You can adjust the size and color of your annotations from the annotation toolbar, and you can drag to move your annotations. Don't like the previous change? Hit Command-Z to undo it.
3. Add your John Hancock
If you have a PDF to sign, Preview can save you the maddening steps of printing it out, signing it and then scanning it back to your Mac. On the annotation menu is a button to add your signature to a file. If you have yet to do so, you can click the Create Signature button and sign your name using your Mac's touchpad. Preview then saves your signature for future use. When you add your signature to a document, you can drag to reposition and resize it so it fits just so when you are asked to sign on the dotted line.
4. Share securely
For sensitive documents, Preview lets you password-protect files. To do so, go to File > Export as PDF. Next, click the Show Details button, check the box for Encrypt, and enter a password.
5. Lock that doc
In addition to password protection, Preview lets you lock files so that others can't make changes without duplicating the file first. To lock a file, click the little down-arrow button next to the file name at the top of the window and check the box for Locked. Others will only be able to view or duplicate the file, but you will still be able to unlock it to make changes.
6. Add, remove, reorder pages
For multipage PDFs, you can drag a PDF from Finder or your desktop to the sidebar of thumbnails in Preview to add a new page or pages to the PDF. Going the other way, you can highlight a thumbnail and go to Edit > Delete to remove it or Command-X to cut it. Lastly, you can highlight and drag and drop a page or pages in the sidebar to change their order.
7. Remove background from an image
Preview took the magic wand tool from Photoshop and renamed it Instant Alpha. No matter the name, the tool remains the same: it lets you highlight the background of an image (PNG or TIFF but not JPEG) to delete the background, with the app smartly detecting the edges between background and foreground object.
To use Instant Alpha, click the toolbox button to open Preview's annotation toolbar. Next, click the magic wand button on the left side of the toolbar, and then drag your cursor across a portion of the background image. Preview should detect the edges of that area and highlight that portion of the background. If not, just try again until it does, although it won't work with images that lack clear edges. With the area selected as you wish, hit the Delete key to remove the background. If the foreground object is large and runs to the edge of the image, you may need to repeat this process for the different segments of the background to remove all of it.
8. Create new file from clipboard
If you have an image and want to perform a quick crop, you can use Preview instead of opening Photoshop, Photos or another proper photo editor. Just highlight a section of the image you want, hit Command-C to copy that selection and then go to File > New from Clipboard. This feature also works with an image you copied to your clipboard from the web, creating a quick way to open up images in Preview.
9. See revision history
Preview borrows a trick from Time Machine to help you reverse course and return to an earlier version of your document. Go to File > Revert To > Browse All Versions and you can spin through the Time Machine-like carousel to revert to an earlier version of your document.