I cannot edit the content in the PDF form i created before

[Helen277673513r11]

I used to have a couple of files using PDF form to create cells so that I can edit the content easily. But I don't know why now I cannot edit them, I can place the cursor to the cell but it just cannot add or erase anything. no warning, no reminder,..... anyone can help please? 

[try67]

Are you trying to edit form fields, or the static contents of the file? What tool are you using?

[Thom Parker]

Can you post the form?

[Lukas Engqvist]

What operating system are you using? If you accidentally open a PDF form with Preview it will corrupt the document even if you do not conciously do anything, no warning given. I would suggest having a .zip of the PDF so that if you accidentally open the file you can restore it.
I also have experience where accessing on a server can change files, compying that file to a local drive has solved the issue. (Note that with newer operating systems Windows and Mac will default to synchronising the dexktop to OneDrive or iCloud,.. which also can also cause problems)