So you have to fill out a form, but you have only a pdf file of the form. You could print it out and use a typewriter to fill it in. You could get Adobe's Acrobat software to edit the pdf file. You could convert the pdf file to postscript with pdf2ps and edit the postscript. Or you could use TeX: include the pdf file as a graphic using TeX's \special command, and then overlay the text you want.

Here's how. Convert the pdf to ps (use pdf2ps, for example). Now suppose the ps file is named myform.ps. If the pdf file has more than one page you should edit out the pages you don't want from the file myform.ps. (Typically, the start of each page is indicated by something like "%%Page: 1 1", "%%Page: 2 2", etc. If all you want is page 3, delete everything from "%%Page: 1 1" up to (but not including) "%%Page: 3 3" and everything from "%%Page: 4 4" up through the last page. Keep whatever denotes the end of the last page; for example, if each page ends with "%%PageTrailer", keep the last occurrence of "%%PageTrailer", and keep the stuff that follows it to end the file. Now change "%%Page: 3 3", which you kept, to "%%Page: 1 1". Also, somewhere there may be an indication, such as "%%Pages: 5", of how many pages were in the original ps file (in this case "5" pages). Change that to "%%Pages: 1".)

Now make a plainTeX file to fill in the form. Here's an example:
\nopagenumbers
\hsize=7in % This sets the width of the page.
% It's not very important, but if it's set
% too small, you'll get overfull hbox alerts,
% and the concomitant black boxes marking them.

% This defines the command used to include the ps file.
\def\theform#1#2#3{\noindent\vbox to0in{\vskip#2in
\hskip#1in\special{psfile="#3"}
\vss}}

% This defines the command to place text over the form,
% thereby filling it out.
\long\def\putit#1#2#3#4{\noindent
\vbox to0in{\vskip#2in\hskip#1in\vbox{\hsize#4in #3}\vss}
\vskip-\baselineskip}

% Here's an example of using the command \theform. The last parameter
% is just the filename (pathname, actually) of the ps file.
% This ps file draws a graphic (the graphic is the form
% you want to fill out). The first two parameters,
% (-1.1, 10.3), specify the coordinates where TeX will place
% the lower left corner of the graphic. The point (0,0) is
% one inch from the top of the page and one inch
% in from the left edge of the page. So in this example,
% the lower left corner of the graphic will be placed
% -1.1 inches left of this (0,0) point, and 10.3 inches
% below it. This would make sense if, say, the graphic
% is about a page long (anything much less
% than 10.3 would allow the top of the graphic to
% run off the top of the page), and indented
% more than you want (so the -1.1 pulls the graphic
% over, cancelling the excess indentation).

\theform{-1.1}{10.3}{myform.ps}

% Now each place you want to enter information, use the
% command \putit. Here the text "June 6, 1944" is put
% starting at TeX page coordinates (4.55, 1.2) (in inches).
% The last parameter, 1.5, specifies the width in inches of the
% \hbox the text is inserted into. If you use too small a value,
% you'll get an overfull \hbox alert, and possibly multiple lines
% where you might have wanted only one line (these come from
% trying to squeeze a too-long line in a narrow box, with the
% result that the long line is broken into two or more shorter
% ones). There seems to be no harm in using large values,
% so just pick a value large enough that the text will obviously
% fit. If however you have a multi-line block of text to fit
% in a box on the form, use a value a little less than the
% width of the box. This will cause the text to break into
% lines of the needed width. Note that the text specified in
% the third parameter of \putit is always regarded as the
% start of a new paragraph, so it's preceded by a paragraph
% indentation. You can either account for this by setting the
% first two parameters appropriately, or you can cancel the
% indentation with \noindent, as in "\noindent June 6, 1944".

\putit{4.55}{1.2}{June 6, 1944}{1.5}

\bye