PDFs that open in unexpected apps — a browser instead of a desktop viewer, or an app that doesn't handle them well — are a file association problem. Your operating system maintains a list of which application should open each file type, and that list can get changed by software installations, updates, or settings without you noticing. The fix is straightforward.
How Default Apps Get Changed
Installing a new application often changes file associations without prominently announcing it. Adobe Acrobat Reader, browser updates, and various PDF tools frequently claim the .pdf file association during installation — sometimes via a checkbox buried in the installer that's checked by default. One day PDFs open in Preview; after an Adobe Reader update, they open in Reader instead.
Operating system updates can also reset file associations to system defaults, undoing your preferences. And on some systems, different PDF viewers compete to be the default, with each update re-claiming the association.
How to Change the Default PDF App on Mac
Find any PDF file in Finder and right-click on it. Select "Get Info" (or press Command-I). In the Info panel, find the "Open With" section and click the dropdown. Choose the application you want — Preview, Adobe Reader, or any other PDF viewer installed on your Mac. Then click "Change All" and confirm. This applies your choice to all .pdf files, not just this one.
If the app you want doesn't appear in the dropdown, click "Other" and navigate to the application in your Applications folder. After selecting it, click Change All to make it the system-wide default.
How to Change the Default PDF App on Windows
Right-click any PDF file in File Explorer and choose "Open with" → "Choose another app." Select your preferred PDF viewer from the list, check the "Always use this app to open .pdf files" box, and click OK. Alternatively, go to Settings → Apps → Default Apps, scroll down to "Choose default apps by file type," find .pdf in the list, and select your preferred application.
On Windows 11, the path is slightly different: Settings → Apps → Default Apps → search for "pdf" and set the default from there. Windows sometimes shows an extra confirmation step asking why you're changing the default — click through it to apply your choice.
PDFs Opening in a Browser Instead of a Desktop App
Chrome and Edge both handle PDFs natively in the browser, and both can claim the PDF file association. If you'd rather PDFs open in a desktop application, the fix is the same as above — set your preferred desktop viewer as the default in your OS settings.
Alternatively, you can control this from within the browser. In Chrome, go to Settings → Privacy and security → Site settings → Additional content settings → PDF documents. Toggle "Download PDFs" on, and Chrome will download PDFs instead of opening them in the browser tab, letting your default desktop app handle them.
When Specific PDFs Open in the Wrong App
Sometimes the global default is set correctly but specific PDFs still open in the wrong application. This can happen if an individual file has application-specific metadata embedded in it, or if the file was downloaded by an application that associated it with itself. Right-clicking the specific file and choosing "Open With" → your preferred app handles the individual case without changing the system default.
Choosing the Right Default PDF Viewer
For most users, the built-in OS option is sufficient: Preview on Mac, Edge's built-in viewer on Windows. These open quickly, handle standard PDFs correctly, and don't require installation or maintenance. Browser-based tools like WukongPDF handle editing, compressing, signing, and converting — tasks the built-in viewer can't do — without needing to be the default file opener.
Adobe Reader is worth installing if you regularly work with PDFs that have complex form fields, certified signatures, or advanced interactive features that simpler viewers don't fully support. Otherwise, the default system viewer plus a browser-based tool for more complex operations covers most needs without the overhead of another installed application.