In Windows, you can associate any program with a file type, even if it cannot open it. If you do make an association there is a high likelihood that a Windows Update will remove the file association, presumably for the joy of doing so.
I have Adobe Reader as default, this suits me very well because if I right-click on a pdf the context menu allows me to select Open with > LibreOffice Draw or any one of a number of different programs. I recommend having your set up the same way. I believe the Draw was added to the Open With menu by Windows after I opened a pdf from within Draw, File > Open.
If you insist on having Draw the default (I strongly advise against this) you can do this relatively easily but not quickly. Open File Manager (Win+E), navigate to a pdf file, right-click it and select Open With > Choose another app.
A new dialog will open titled How do you want to open this file? , if you can see LibreOffice Draw in the list, then select it and tick the box labelled Always use this app to open .pdf files.
Otherwise, scroll down to the bottom and click on the link More apps ↓, if you can see LibreOffice Draw in the list, then select it and tick the box labelled Always use this app to open .pdf files.
Otherwise, scroll down to the bottom and click on the link Look for another app on this pc, navigate to C:\Program Files\LibreOffice\program\sdraw.exe (if you can’t find it here then you have answered your second question; you have installed a 32 bit version of LO) and click OK, tick the box labelled Always use this app to open .pdf files.
Open Writer or any other LO program, click Tools > Options > LibreOffice > Advanced, can you see a Java version installed? If not then click on Add, a new dialog will open, navigate to C:\Program Files\Java and click Select Folder. Wait a little time, eventually you should see a radio button with the java version next to it, mine is jre1.8.0_271