Lock pdf file?

[#1]

I have several editable pdf files (forms) that I have downloaded, completed and sent off.
I keep copies for my records but want to be able to prevent each file from being altered. I haven't found an option in pdf Reader or pdf XChange Viewer and can't afford Adobe Acrobat.
Does anyone have a possible solution to this, please?

[#2]

Something that would convert a pdf file to a jpeg perhaps?
One or two in Google!
Or, set the pdf to Read Only in its Properties.

[#3]

Not tried this myself but just found it on google :
https://ra5-downloads.phpnuke.org/en/c156342/pdf-locker-free-download-full-review

[#4]

In PDF X-Change Editor (which is v3 of PDF X-change viewer), there is File -> Document Properties, and there you can set password security to restrict opening or editing the file.

[#5]

Thanks for the replies...
@NedLudd Thanks for the suggestion but I do prefer to keep as a pdf file. Couldn't see any options within Acrobat Reader or XChange Viewer to set to Read Only.
@gtowen I saw that when I did a search - just wondered if anyone had any other ideas
@ejs I didn't realise there was another version available. Have downloaded it to try. Didn't really want to password protect as it would still be editable. No-one else will have access to the files but accidents can happen and I have made changes I didn't necessarily want to in error then save them before so just wanted to avoid that happening again. All in all it's not worth too much effort in solving and I can easily save the password in the filename if I go that route.
Thanks again for all the suggestions.

[#6]

Mav, you set the Read Only attribute in the Properties for the filename! i.e. locate the file with Filemanager and right click on it, select Properties and Bob's yer uncle!

[#7]

Doh
Obvious now it's been pointed out. Thanks - that should be enough for my purposes.

[#8]

A 'Senior Moment' perhaps! 

[#9]

One of many today

[#10]

Setting it "Read Only" in Properties doesn't "prevent" the file from being changed, merely mitigates.
I certainly had thought you were referring to stopping someone altering it intentionally not unintentionally as now seems to be the case.
Another way - somewhat perverse I must admit but it'll work - is to change it to a jpg, then printing the jpg as a pdf file. You then get a pdf file that cannot readily be altered since it contains a graphic image not text. Quality will suffer though.

[#11]

@drj But at least it will prompt me to cancel any changes if there had been any.
Your last suggestion wouldn't be worth the effort but worth bearing in mind.