By using a controlled access software program for all the PPMs, then it becomes a controlled document, or at least you can argue that it is.
But, if you don't use a program or some other kind of secure way (like read-only without a password), then it is not.
The rule of thumb for whether or not something should be a controlled document is if changing it or using an outdated version could potentially have a serious effect on the quality or safety of your business, then it should be controlled.
There are ways to make controlled documents a little flexible, by saying that the lubricants used must be on the approved list for example, which does not have to be controlled, things like that. But the basic process for each PPM really should be cast in stone.
That's my $0.02 worth.
"...everything can be taken from a man but one thing: the last of the human freedoms--to choose one's attitude in any given set of circumstances, to choose one's own way." Viktor E. Frankl
"Life's like a movie, write your own ending." The Muppets