Adobe Reader is THE program to use when reading pdf files. When you access a site that provides “fillable” files, (pdf files you can use to fill in the blanks, for example), the file downloads and opens in Adobe Reader, allows you to fill in the blanks, but will not save the completed form. Why not?
The web site you are downloading from uses an updated format. The first item to check is the version of Adobe Reader you are using. If you do not have the latest Reader, (Reader DC), then download it by googling Adobe Reader and follow the links to the site.
To check the version of Reader, open the program and click on Help, then About Adobe Reader. A window will pop up showing you which version you have. Note: the website you are downloading from is responsible for providing the fillable file types. You will not be able to fill in a pdf file that is not created as “fillable.”
Once installed you should be able to download, fill out and save the completed document on your computer.