Do you have your windows explorer set to show full filenames and show file extensions?

When a file "acts strangely" it's a good idea to suspect malware, although there are a few non-sinister things that can cause unusual behavior. Although not used a lot recently, malware still sometimes disguises itself with "double dot" filenames. A file named document.pdf.exe may display as a pdf but actually is an executable file. With the "show all file extensions" turned on, you should see both extensions. Until you solve your problem, you may also want to turn on display of all hidden and system files.

[Probably not directly applicable here, but Microsoft has been known to us a similar trick with files with names that appear to start with a "~". The beginning ~ character frequently indicates a temp file, but in this usage a different (unknown except by Microsoft) character that defaults to display the ~ because it's unrecorded in character maps was used. Explorer couldn't delete those, because you can't "spell the filename" correctly. At a command prompt, you could, however, rename ~file.xxx using the wild-card ? and the command "REN ?file.xxx afile.xxx" and then delete afile.xxx.]

PDF files can be "locked" to prevent changes, and a locked file may block deletion. For this reason, most sites that post them in locked form block saving of those PDFs, but allow you to open them and "save a copy." The copy will generally not be locked.

Most people have Adobe Reader rather than Adobe Acrobat, and I don't recall whether Reader even shows whether a file is locked(?). Acrobat generally refers to the expensive versions that can create and edit pdf files, although Adobe had changed the name of the "editing" versions seven times the last time I looked, about two years ago. It's probably been changed again since then.

John