Problem viewing silhouette plot in Adobe Acrobat

I'm having a problem viewing a silhouette plot created as a pdf file  
in Adobe Acrobat.  The code below produces a silhouette plot which,  
when saved as a pdf file, has bars that are not visible in Adobe  
Acrobat. A similar problem was noted earlier in the archives (from  
which I took this code) but for a png file. The archive emails  
suggested it may have been a resolution problem however did not have  
a simple workaround, which I assume may exist.

Note also that the bars of the silhouette plot below are viewable in  
the MAC "preview" app.

    library(cluster)
    nmat <- matrix(rnorm(2500*300), ncol=300, nrow=2500)
    rmat <- matrix(rchisq(1000, 300, 50), ncol=300, nrow=1000)
    mat <- rbind(nmat, rmat)
    pr <- pam(mat, 2)
    plot(silhouette(pr))

Any help would be appreciated,

Many thanks,
Joe Retzer

I'm having a problem viewing a silhouette plot created as a pdf file in 
Adobe Acrobat.  The code below produces a silhouette plot which, when 
saved as a pdf file, has bars that are not visible in Adobe Acrobat. A 
similar problem was noted earlier in the archives (from which I took 
this code) but for a png file. The archive emails suggested it may have 
been a resolution problem however did not have a simple workaround, 
which I assume may exist.

Note also that the bars of the silhouette plot below are viewable in the 
MAC "preview" app.

     library(cluster)
     nmat <- matrix(rnorm(2500*300), ncol=300, nrow=2500)
     rmat <- matrix(rchisq(1000, 300, 50), ncol=300, nrow=1000)
     mat <- rbind(nmat, rmat)
     pr <- pam(mat, 2)
     plot(silhouette(pr))

Any help would be appreciated,

Many thanks,
Joe Retzer