Mohammad from Denmark writes: I have a problem of making PDF file association in Windows 7, even though I have the Adobe Acrobat reader installed. Removed it and installed again, but still the problem persists. Maybe you have a trick to fix it, in that case would be nice to share it with me and the others.
Windows 7 features several different ways to associate files with specific programs, so if one of them doesn’t take for some reason, there are still other options available to associate PDF files with Adobe Reader. In this situation uninstalling and reinstalling Adobe is always a good idea, as it removes all old settings, but since the reader already went through that process we’ll start with changing the PDF file association instead.
To force Windows to always open a PDF file with Reader, open the Start menu and click the “Default Programs” button on the right side of the menu.
Click the link labeled “Associate a File Type or Protocol with a Program” at the center of the Default Programs window.
Scroll down through the list of file extensions and highlight the “pdf” entry, and then click the click “Change Program” button at the top-right corner of the window.
Click “Browse” and select the “Adobe Reader” program. Repeat the file association process with the “pdfxml” file extension.
Click the “Close” button and Reader should now automatically open PDF files. If it doesn’t, right-click any PDF file on your computer and select the “Open With” option.
Click “Browse” and select the Adobe Reader program, but make sure the check box at the bottom of the screen labeled “Always use the selected program to open this file” is checked. Click “OK” to associate PDF files with Adobe Reader.
If these Windows options don’t do the trick, you can also associate PDF files with Reader directly In the Adobe Reader program itself. Load a PDF file in Reader and open the “Edit” menu at the top of the screen. Select the “Preferences” option at the bottom of the menu.
Navigate to the “General” tab at the left side of the screen and click the “Select Default PDF Handler” button at the bottom of the window.
Choose your version of Adobe Reader from the drop down menu and then click the “Apply” button.
In most cases the above mentioned steps should fix the problem, but if Windows still isn’t remembering that PDF files should be opened with Reader you likely have an issue in the registry that needs to be manually changed. Keep in mind that changing registry settings can cause serious problems, so be sure to back up any important data before proceeding. To access the registry editor, open the Start menu, type “regedit” in the search box, and click the “regedit” icon.
Open the folder labeled “HKEY_CLASSES_ROOT” at the top of the list of registry folders.
Scroll through the list of file types and click the entry labeled “PDFFile.OpenDocuments.”
Locate the entry labeled “Default” in the panel at the right side of the window. Right-click the entry and select “Modify.”
Type “Adobe Acrobat Sharepoint OpenDocuments Component” (without the quotation marks) in the text box and click “OK.” Restart the computer to save the change.
In the event your computer still won’t associate PDF files with Adobe Reader, you could instead download and install a different PDF reading program – such as PDF X-Change Viewer , Fox It , or Sumatra PDF  – and then associate PDF files with that program instead.